A retrospective analysis of 27 patients seen at UCLA in whom transitional cell carcinoma of the bladder developed at age forty or younger was conducted. Forty-one per cent had tumors manifesting known characteristics of aggressive and lethal potential: high histologic grade, muscular invasion, severe epithelial atypia, and frequent multifocal recurrence. One of these patients had extensive pelvic node metastases. The time from the first symptom, usually hematuria, to endoscopic diagnosis exceeded six months in 8 patients. The results of segmental cystectomy were very poor, in the absence of narrowly defined criteria for selection of this mode of therapy. No evidence was found to suggest that transitional cell carcinoma of the bladder in patients forty years of age and under differs clinically or morphologically from that of older patients. Treatment should be determined by the stage of the tumor and other indices of potential lethality.