Objectives:We analyzed the clinical outcomes of instillation therapy with Bacillus Calmette-Guerin (BCG) to treat carcinoma in situ (CIS) and searched for prognostic factors that could predict disease progression. Methods: Between January 1995 and January 2001, 185 patients (male, 155; female, 30) diagnosed with bladder CIS underwent weekly BCG instillations (80 mg of Tokyo 172 strain) for eight weeks. Primary, concomitant, and secondary CIS was found in 62 (33.5%), 60 (32.4%) and 63 (34.1%), patients, respectively. Seventy-five (40.5%) and 64 (34.6%) patients had limited and extensive CIS, respectively. The median follow up period was 37.5 months (range 4-95 months). Results: The overall complete response rate was 86.5%. The five-year progression-free survival rate was 78.5%. Several factors, such as age (<60 or Ն60 years), gender, previous transurethral resection, type of CIS, and CIS extension (three or more positive sites out of four to six biopsy sites was defined as extensive), were examined by multivariate analysis to predict progression. The extension of CIS was the only independent prognostic factor. The five-year recurrence-free rate of complete responders (n = 160) was 66.0%. Radical cystectomy was performed in 10 patients (6.3%) during follow up incomplete responders, of whom seven had invasive bladder cancer. Extravesical involvement was identified in 30 patients (16.2%) among whom, 21 (11.3%) had upper urinary tract recurrence and nine (4.9%) had prostatic involvement. Conclusion: Therapy with BCG is effective against CIS, the extent of which might be a prognostic factor. Disease progression including extravesical involvement should be carefully monitored over the long-term after BCG therapy.