Patient description: A 17 year old boy complains of vomiting, non-bloody diarrhea, abdominal pain, fever, chills and loss of appetite for the past 3 days. He ate a salad at a restaurant prior to his diarrhea onset. Physical exam was remarkable for pallor, jaundice, and diffuse abdominal tenderness. Lab results were as follows: Hemoglobin: 9.7 g/dL Platelet: 110,000 /cu.mm Creatinine: 3.6 mg/dL blood urea nitrogen (BUN): 73 mg/dL direct bilirubin: 2.4 mg/dL lactate dehydrogenase (LDH): 881 IU/L (normal: 110-265 IU/L) Peripheral blood smear showed a moderate number of schistocytes and helmet cells. Shiga-like toxin-producing E. coli (STEC) stx1/stx2 were found in stools. He has no other underlying disease and he is not on any medications.

Clinical trial eligibility criteria: eligible ages (years): 0.0-31.0,  Kaposiform Hemangioendotheliomas Tufted Angioma Capillary Venous Lymphatic Malformation Venous Lymphatic Malformation Microcystic Lymphatic Malformation Mucocutaneous Lymphangiomatosis and Thrombocytopenia Capillary Lymphatic Arterial Venous Malformations PTEN Overgrowth Syndrome With Vascular Anomaly Lymphangiectasia Syndromes will be strictly limited to children and young adults with vascular anomalies with complications that require systemic therapy for control. Diagnosis: All patients must have one of the following vascular anomalies as determined by clinical, radiographic and histologic (when possible) Kaposiform Hemangioendotheliomas with Kasabach-Merritt Phenomenon Kaposiform Hemangioendotheliomas without Kasabach-Merritt Phenomenon Tufted Angioma with Kasabach-Merritt Phenomenon Tufted Angioma without Kasabach-Merritt Phenomenon Capillary Lymphatico-Venous Malformation (CLVM) Venous Lymphatic Malformation (VLM) Microcystic Lymphatic Malformation (MLM) Multifocal Lymphangiomatosis and Thrombocytopenia (MLT)/Cutaneovisceral Angiomatosis and Thrombocytopenia (CAT) Capillary Lymphatic Arterial Venous Malformations (CLAVM)  Dental braces or prosthesis only if it interferes with radiologic analysis of vascular anomaly Concurrent severe and/or uncontrolled medical disease which could compromise participation in the study (e.g. uncontrolled diabetes, uncontrolled hypertension, severe infection, severe malnutrition, chronic liver or renal disease, active upper GI tract ulceration) Chronic treatment with systemic steroids or another immunosuppressive agent. Patients with endocrine deficiencies are allowed to receive physiologic or stress doses of steroids if necessary. Patients with the diagnosis of a vascular tumor (KHE, TA) can be on a weaning dose of steroids Patients who require medications that inhibit/induce CYP3A4 enzyme activity to control concurrent medical conditions Known history of HIV seropositivity or known immunodeficiency. Testing is not required unless a condition is suspected Impairment of gastrointestinal function or gastrointestinal disease that may significantly alter the absorption of sirolimus (e.g. ulcerative disease, uncontrolled nausea, vomiting, diarrhea, malabsorption syndrome or small bowel resection). A gastric tube or nasogastric tube is allowed Women who are pregnant or breast feeding Males or females of reproductive potential may not participate unless they have agreed to use an effective contraceptive method during the period they are receiving the study drug and for 3 months thereafter. Abstinence is an acceptable method of birth control. Women of childbearing potential will be given a pregnancy test within 7 days prior to administration of sirolimus and must have a negative urine or serum pregnancy test Patients who have received prior treatment with an mTOR inhibitor Patients unwilling or unable to comply with the protocol, or who in the opinion of the investigator may not be able to comply with the safety monitoring requirements of the study