This case was reported in a literature article and described the occurrence of hepatic vesicular steatosis in a 9-month-old male subject who was vaccinated with hepatitis B vaccine recombinant (Hepatitis B vaccine, manufacturer unspecified). The subject was born after a full-term gestation and was delivered vaginally. Apgar scores were 8 and 9 at 1 and 5 minutes, respectively. Results of routine newborn metabolic screening were normal. Hepatitis B vaccine was administered before discharge. The subject's medical history included a respiratory infection at 8 months of age, a fall with bruise on forehead 3 days before admission. His only medication was acetaminophen for 1 day. The subject had been healthy, with normal growth and development. Childhood immunizations were reportedly up to date. He had no known allergies. He lived with his mother, who had a respiratory symptoms for 1 week and received azithromycin. He did not attend day care and did not have known travel or toxic exposures, including mushroom ingestion. His mother had had seizures in childhood. After birth, at discharge from hospital, the subject received 1st dose of Hepatitis B vaccine (unknown route, unknown injection site, unknown lot number). At 9-months of age, the subject was admitted to the hospital because of fever, diarrhea, and liver failure. The patient had been well until 2 days before admission, when lethargy, irritability, rhinorrhea, intermitted vomiting, and non bloody diarrhea developed, with decreased oral intake. The next day, the temperature reportedly increased to 38.1 deg C. He was seen in a health centre, the temperature was 37.6 deg C, and the physical examination was normal. Acetaminophen and a pediatric oral electrolyte solution were prescribed for presumed viral gastroenteritis, and he was sent home. The next day, symptoms worsened; in the evening, the was brought to the emergency department at this hospital. On examination, he was listless, intermittently restless, and whimpering. The weight was 8.9 kg (25th to 50th percentile for age), the temperature 37.6 deg C, the blood pressure 95/46 mmHg, and the pulse 133 beats per minutes; the respiratory rate was 26 breaths per minute, and the oxygen saturation 100% while he was breathing ambient air. The conjunctivae were pale and non-icteric. There was a bruise above the left eye. Bowel sounds were diminished, and the abdomen was soft, without tenderness or distention. The liver edge extended 4 to 5 cm below the costal margin. The remainder of the examination was normal. The levels of magnesium, C-reactive protein, amylase, lipase, iron, total iron-binding capacity, and lactate were normal. Urinalysis was normal, and screening of the urine for toxins was negative. A radiograph of the abdomen showed a nonspecific bowel-gas pattern, with no evidence of free air or bowel obstruction. Intravenous fluids were administered. Computed tomograph (CT) of the brain without contrast administration, was normal. Ultrasonography of the abdomen showed a slightly echogenic liver and pericholecystic fluid, with no evidence of intrahepatic or extrahepatic biliary-duct dilatation. After sedation with midazolam, CT of the chest, abdomen, and pelvis with the administration of oral and intravenous contrast material showed a marked enlarged liver with diffuse hypoattenuation consistent with fatty infiltration and pericholecystic fluid. The CT examination was otherwise normal. The patient was admitted to the pediatric intensive care unit. A central line was placed in the femoral vein; cardiovascular and respiratory variables were monitored continuously; and oral intake was restricted. N-acetylcysteine and ceftriaxone were given; ranitidine and vitamin K were administered; lorazepam and morphine were given for agitation. The level of serum acetaminophen was less than 10.0 mg per liter (reference range, 10 to 25); none was detected in the urine. During the first 2 days, testing for serum heterophile antibodies, Epstein-Barr virus (EBV), and hepatitis A and C viruses was negative, as were tests for respiratory viruses (adenovirus, influenzainfluenza A and B viruses, parainfluenza, and respiratory syncytial virus antigen) performed on a nasal specimen; testing for hepatitis B virus surface antibody was positive and surface antigen negative. Cultures of the blood, urine, and stool were sterile. During the next 2 days, hypotension and hypoxemia developed, with systolic blood pressure of 60 to 70 mm Hg, diastolic pressure of 30 to 40 mm Hg, and an oxygen saturation of 88% while the patient was breathing ambient air; he became increasingly lethargic, with sluggish papillary reactions. Oxygen by nasal cannula and normal saline boluses were administered; blood pressure returned to normal but then decreased; dopamine and norepinephrine were administered. Electrocardiography revealed a sinus rhythm of 153 beats per minute, with an incomplete right bundle-branch block and right ventricular hypertrophy; echocardiography was normal. CT of the head was normal. Testing for syphilis, antibodies to toxoplasma and parvovirus B19, bartonella (immunoglobulins IgG and IgM), varicella-zoster virus (IgG), and cytomegalovirus (CMV) antigen was negative. On the third day, red cells were transfused; the hematocrit increased to 31.4%. The liver was to 8 cm below the costal margin, extending to the umbilicus and firm to palpation. Additional test results were pending. A diagnostic procedure was performed. To establish the final diagnosis, several tests were performed such as abdominal ultrasound, computerized tomogram abdomen, liver biopsy, magnetic resonance of brain, analysis of screening panel. After differential diagnosis, a fatty acids oxidation defect with a carnitine acylcarnitinetranslocase (CACT) deficiency was noted. The final diagnosis was diffuse microvesicular and macrovesicular hepatic steatosis due to CACT deficiency. Laboratory tests were performed at admission for three days and relevant results included the following: Hematocrit: 20% (33-39); Hemoglobin: 7.4 g/dl (10.5-13.5); Aspartate aminotransferase: 3718 IU/L (9-80); Alanine aminotransferase: 1920 IU/L (10-55); Lactate dehydrogenase: 2031 IU/L (110-210); Uric acid: 17.8 mg/dl (1.6-8.5); Creatine kinase isoenzymes: 18.5 ng/ml (0-6.9); Creatine kinase: 1407 IU/L (60-400); Ferritin: 1230 ng/ml (30-300); Triglycerides: 340 mg/dl (40-150). The subject was treated with paracetamol (Acetaminophen), hydration therapy (oral rehydration solution), N-Acetylcysteine, ceftriaxone, ranitidine hydrochloride (Ranitidine), vitamin k, lorazepam, morphine, oxygen, intravenous fluid(s) (Fluid bolus), dopamine, noradrenaline (Norepinephrine), Ursodesoxycholic acid, diet and carnitine. This patient was treated by removing long-chain fats from the diet and supplementing his diet with oral carnitine. Ursodeoxycholic acid was also added by the gastroenterology service to reduce absorption of fats. The long-term strategy consisted of a multidisciplinary team approach with a coordinated care clinic, a diet aimed at his enzymatic deficiencies, and oversight of biochemical genetics. The key will be avoidance of fasting, with frequent meals and a special diet with restriction of long-chain fatty acids.