From 1975 through 1983, 69 patients with hereditary angioneurotic edema were treated with danazol, a semisynthetic anabolic steroid. Hematuria developed in 13 of these patients (19 per cent). Careful evaluation of the genitourinary tract showed the presence of a distinct form of hemorrhagic cystitis in 10 patients. Clinically, 9 patients with microscopic and 1 with intermittent gross hematuria. Irritative bladder symptoms were reported by 2 patients. Neither the dose nor duration of danazol therapy correlated with the severity of the cystoscopic or pathological findings. Cystoscopically, a rather nonspecific pattern of erythema, submucosal telangiectasia and neovascularity was observed. Histopathologically, bladder biopsy typically showed numerous dilated submucosal vessels with hemorrhage, mucosal ulceration and occasional inflammatory cells. These changes regressed in all but 1 patient when danazol therapy was discontinued. Further studies are needed to elucidate the role of danazol itself or of the danazol-hereditary angioneurotic edema interaction in the pathogenesis of these abnormalities.