Background: Complicated Kaposiform hemangioendothelioma (KHE) and Tufted angioma (TA) is rare vascular tumor and difficult to treat. There has several therapeutic options but the standard therapy for KHE has not proven. Method: We prospectively evaluate a series of four patients with complicated KHE and TA treated with oral sirolimus, mTOR inhibitor, in single-center of a Korean university-setting hospital with multidisciplinary cooperation (Vascular Anomalies Clinic). Dosage of sirolimus was 0.8 mg/BSA per dose twice a day, with adjusting the target drug levels of 10 to 15 ng/mL. The subjective response (quality of life and lesion volume or size) was check every 3 month and imaging response (MRI or USG finding) was checked every 6 month. Results: These patients represented dramatic improvement in clinical manifestations with tolerable side effects (Table 1). Conclusion: We suggest sirolimus may be an effective and well-tolerated therapeutic modality in selected patients with complicated KHE and TA, especially with lymphatic components dominance.