Ninety-one cases of intravascular papillary endothelial hyperplasia were studied clinically and histologically. This peculiar benign process, occasionally resembling hemangiosarcoma, was subgrouped in the following categories: a pure form that occurs within a dilated vascular space (30 cases), a mixed form that appears as a focal change in a hemangioma (55 cases), a third form (six cases) that belongs to neither of the first two. In the pure form, the lesions were most frequently situated in the subcutis of fingers (14 cases), of the head and neck (seven cases), and in the region between the elbows and hands (six cases). In the mixed form, half of the accompanying hemangiomas were intramuscular in no particular predilective sites. Papillary proliferation of endothelial cells was commonly found to be closely associated with thrombotic material and seemed to be an unusual feature of a thrombus undergoing organization.