We studied 19 consecutive patients with biopsy proved carcinoma in situ of the bladder who had received 8 weekly doses of 30 mg. intravesical mitomycin C. Followup with cystoendoscopy, biopsy and cytology studies every 3 months ranged from 6 to 48 months, with an average of 21 months. A complete initial response was achieved in 15 patients, 11 of whom have remained free of tumor with no further therapy. The 4 patients who failed to respond to initial therapy subsequently were free of tumor after further intravesical mitomycin C (2), excision (1) or doxorubicin (1). Three patients died of unrelated diseases. Only 1 patient has had metastatic transitional cell carcinoma. No patient had undergone cystectomy or radiotherapy. These data indicate that mitomycin C achieves a high initial response in patients with carcinoma in situ of the bladder and maintains this control in many patients for months or years.