Sirolimus (Rapamune), a mammalian target of Rapamycin (mTOR) inhibitor, which has been used extensively in children following solid organ transplantation, has been demonstrated to have anti-angiogenic activity in pre-clinical models. Limited experience suggests that it may have application to the treatment of vascular lesions. We describe our experience with a 1-year-old female with a kaposiform hemangioendothelioma and Kasabach-Merritt phenomenon who had rapid and dramatic response to sirolimus (0.1 mg/kg/day). This case provides further rationale for clinical trials of sirolimus in the treatment of vascular lesions.