Seventy patients were given courses of intravesical mitomycin C for residual transitional cell carcinoma of the bladder following partial resection or biopsy. The patients were reassessed 3 months after the initiation of treatment by cystoscopy and cytology from cystoscopic urines and bladder washings. Twelve had no visible cancer at cystoscopic study but had positive urine cytologic findings. The incidence of tumor recurrence, cystectomy, radiotherapy, and deaths due to bladder cancer for this group of cytologically positive partial responders was analyzed. Thirty-three percent (4/12) required cystectomy, none underwent radiation therapy and none died of bladder cancer. These outcomes were compared with that of complete responders (negative cystoscopic and cytologic results) and partial responders with visible tumor (reduction by greater than 50%). We conclude that in high-grade carcinomas, particularly carcinoma in situ, positive urine cytologic findings at the initial 3-month follow-up visit following treatment with intravesical mitomycin C is as ominous a prognostic indicator as endoscopic or biopsy evidence of cancer.