Angiogenic activity has been assessed in biopsy specimens from 49 patients with transitional cell carcinoma, benign prostatic hypertrophy and vesicoureteral reflux. Over 95% of the specimens containing malignant and premalignant transitional epithelium stimulated capillary proliferation on rabbit iris. In contrast, less than 10% of normal tissues had this effect. Sixty-one per cent of specimens with inflammatory round cells were angiogenic but this was reduced to 10% by preincubation with rabbit antihuman lymphocyte serum. Cystitis cystica also induced capillary proliferation even if incubated with the antilymphocyte serum. Angiogenic capacity may be an early marker of preneoplastic and neoplastic lesions of the human bladder.