INTRODUCTION: Micropapillary carcinoma is a rare pathologic variant of urothelial cell carcinoma. Intravesical bacillus Calmette-Guerin (BCG) has been reported to be ineffective and to entail an increased risk of development of non-organ-confined, metastatic disease. We assess the treatment response and disease progression in patients with micropapillary carcinoma of the bladder. MATERIALS AND METHODS: The study comprised 18 patients with micropapillary carcinoma of the bladder who underwent transurethral resection of a bladder tumor and multiple random biopsies between 1997 and 2003. We retrospectively analyzed treatment response and clinical and pathological cancer evolution related to cancer stage and the percentage of the micropapillary component of the cancer. RESULTS: Seven of the 18 patients (38.8%) had carcinoma in situ. At diagnosis, 8 of the 18 patients had non-muscle-invasive bladder cancer; 6 of these patients were treated with intravesical BCG therapy and were alive and free of disease at a median follow up of more than 5 years. Ten of the 18 patients had muscle-invasive bladder cancer; 8 of these patients underwent radical cystectomy, and 7 of the 8 patients (87.5%) had non-organ-confined disease in cystectomy specimens. Seventy percent of patients with muscle-invasive disease at diagnosis had a micropapillary carcinoma component of more than 50% in transurethral resection of the bladder specimens, compared with only 25% of patients with non-muscle-invasive disease. Patients treated successfully with intravesical BCG therapy had a low micropapillary carcinoma component. The 5-year disease-specific survival rate was significantly lower in patients with muscle-invasive disease (30%) than in patients with non-muscle-invasive disease (87.5%) after a median follow up of 52 months (p = 0.001), and it was also significantly lower in patients with a high percentage of the micropapillary component of the carcinoma. CONCLUSIONS: This retrospective study of 18 patients with micropapillary carcinoma of the bladder suggests that tumor stage and patient outcome may be related to the percentage of the micropapillary component of the carcinoma. Radical surgery is mandatory in muscle-invasive disease, even though patients with lymph node involvement die from the disease. In non-muscle-invasive disease and in the absence of associated carcinoma in situ, intravesical BCG treatment may be offered when the micropapillary component of the carcinoma component is a small percentage.