Background Vascular anomalies comprise a diverse group of diagnoses. While infantile hemangiomas are common, the majority of these conditions are quite rare and have not been widely studied. Some of these lesions, though benign, can impair vital structures, be deforming, or even become life‐threatening. Vascular tumors such as kaposiform hemangioendotheliomas (KHE) and complicated vascular malformations have proven particularly difficult to treat. Procedure Here we retrospectively evaluate a series of six patients with complicated, life‐threatening vascular anomalies who were treated with the mTOR inhibitor sirolimus for compassionate use at two centers after failing multiple other therapies. Results These patients showed significant improvement in clinical status with tolerable side effects. Conclusions Sirolimus appears to be effective and safe in patients with life‐threatening vascular anomalies and represents an important tool in treating these diseases. These findings are currently being further evaluated in a Phase II safety and efficacy trial. Pediatr Blood Cancer 2011; 57: 1018–1024. © 2011 Wiley‐Liss, Inc.