Technical Abstract: Accurate classification, diagnosis, management, and treatment of vascular lesions in children can be hindered by the wide range of clinical presentations and varying clinical course of these lesions. In this issue of Pediatrics, Marler et al1 report novel findings that elevated levels of angiogenesis-related proteins, specifically high molecular weight matrix metalloproteinases (MMPs) and basic fibroblast growth factor (bFGF), can be detected in the urine of children with vascular anomalies and can mark clinical progression of these lesions. These findings suggest that a noninvasive test can be developed to characterize aggressive vascular malformations and tumors and provide additional evidence that antiangiogenic agents may be useful for the treatment of these lesions.