Vascular malformations, particularly in the absence of cutaneous changes, can be difficult to distinguish from other soft tissue masses in children. We have used technetium-99m-labeled red blood cell scintigraphy to study 47 lesions in 43 children. Thirty-nine lesions showed increased flow and were, therefore, diagnosed as vascular malformations. Subsequent biopsy of 10 of these lesions confirmed that diagnosis. The other 29 lesions with increased flow were followed for 10 months to 5 years and the clinical course was consistent with vascular malformation in every case. Eight lesions showed no increased flow on technetium scan. One of these subsequently proved to be a hemangioma. The others have turned out not to be vascular malformations. Therefore, in our experience, the technetium-99m-labeled red blood cell scan has had 98% sensitivity and 100% specificity in diagnosing vascular malformations in children.