Malignant soft tissue tumors are rare and difficult to diagnose in children. These can initially be misdiagnosed as benign vascular anomalies. Management of these two conditions differs drastically and delay in diagnosis may impact overall survival. To predict qualities that may increase the index of suspicion for malignancy in patients presenting with lesions initially considered vascular anomalies. A retrospective review at a quaternary hemangioma and vascular malformation center of all patients who presented between 2008 and 2016 with an initial diagnosis of a benign vascular malformation, which on further work-up was noted to be a malignancy. Demographics, clinical presentation, and laboratory and radiologic studies were analyzed. Eleven patients were identified; the median age at presentation was 2 months (0-24years). Ten out of 11 lesions had rapid growth, which prompted biopsy. Pain was an inconsistent finding (36%). Tumor markers were positive in only one case. Median follow-up was 3 years (range 6 months to 8 years); seven patients have no evidence of disease, two patients are under treatment for progression or relapse of disease, and two patients have died. Although malignant vascular tumors are rare, a clear index of suspicion needs to be maintained particularly with rapid growth or increasing symptoms. Differentiation of malignant tumor from benign lesions relies on the comprehensive evaluation of clinical manifestations, evolution of the lesion, and abnormal clinical behavior, by an experienced multidisciplinary vascular malformation team. There should be a low threshold for biopsy of unclear vascular lesions.