Initial unsolicited literature report received from a physician on 16-Jan-2018. Abstract: Lichen planus is an inflammatory dermatosis with a prevalence of approximately 1%. Recent meta-analyses show that patients with hepatitis C virus have a 2.5- to 4.5-fold increased risk of developing lichen planus. Lichen planus has also followed vaccinations and has specifically been attributed to the hepatitis B vaccine, the influenza vaccine, and the tetanus-diphtheria-acellular pertussis vaccine. We describe a case of lichen planus in a hepatitis C virus-infected male occurring in temporal association with the administration of the tetanus-diphtheria-acellular pertussis vaccine. The patient's presentation was clinically consistent with lichen planus and confirmed by biopsy. It is likely that many cases of vaccine induced lichen planus have gone unpublished or unrecognized. In areas with high prevalence of hepatitis C virus infection, we may expect to see more cases of vaccine-induced lichen planus especially in light of the updated Centers for Disease Control and Prevention tetanus-diphtheria-acellular pertussis vaccination recommendations. This case serves to educate healthcare providers about vaccine-induced lichen planus and, in particular, the need to counsel hepatitis C virus-infected patients about a potential risk of developing lichen planus following vaccination. We also reflect on current theories suggesting the T-cell-mediated pathogenesis of lichen planus and the role that hepatitis C virus and toxoid or protein vaccines may play in initiating the disease. This case involves a 54-year-old male patient who was vaccinated with a dose of ADACEL (Lot number, expiry date, dose, dose in series, route and site of administration were not reported) on an unknown date. The patient's medical history was not reported but presented with chronic HCV (hepatitis C virus) that was ongoing. Concomitant medications were reported as the patient's daily medications that were, glimepiride 2 mg, metformin 500 mg, loratadine 10 mg, omeprazole 320 mg, simvastatin 40 mg, triamterene-hydrochlorothiazide 75-50 mg daily, ferrous sulfate 325 mg, methadone, and a fluticasone inhaler 100 mg/actuation. As-needed medications included clonazepam 0.5 mg, oxycodone/acetaminophen, and an albuterol inhaler 90 mcg/actuation. On an unknown date, 10 days following the vaccination, the patient experienced intensely pruritic rash on his hands, legs, and trunk, violaceous, flat-topped, 3-to 6-mm discrete and coalescing papules on his dorsal hands, back, chest, abdomen, buttocks, and lower extremities, examination of buccal mucosal surfaces revealed slight hyperpigmentation without evidence of Wickham striae and clinical diagnosis of LP (lichen planus) was confirmed when punch biopsy of a representative skin lesion was obtained. The laboratory data included transaminase elevation (aspartate transaminase (AST): 79 U/L, alanine transaminase (ALT): 56 U/L), elevated alpha- 2-acroglobulins (356 mg/dL), low apolipoprotein A-1 (81 g/L), low albumin (3.4 g/dL), and a microcytic anemia with low serum iron (24 mg/dL). Serum studies were positive for anti-HCV antibody and HCV RNA was elevated at 6.198 log IU/mL. HCV FibroSURE fibrosis score was 0.83 and necroinflammatory activity score was 0.59, indicating fibrosis stage F4, cirrhosis, and moderate necroinflammatory activity. Rapid plasma reagin and human immunodeficiency virus were both negative. The patient was treated with 60 mg KENALOG intramuscularly and topical clobetasol 0.05% ointment. Significant improvement was noted on his 8-week follow-up visit. The patient was recovering from the event. List of documents held by sender: none.