Of 347 patients who received an initial 6-week course of intravesical bacillus Calmette-Guerin for superficial transitional cell carcinoma of the bladder 28 (8%) were treated with another course. Subsequent progression of disease (muscle infiltration, metastasis or local progression) occurred in 13 patients (46%). Of the 15 patients (54%) without progression 10 (36%) had a complete response and 5 (33%) had new tumors, and they were rendered free of disease after transurethral resection. The median duration of response to course 1 of bacillus Calmette-Guerin was shorter for patients with disease progression after course 2 than for those with no progression (15 and 27 months, respectively, p equals 0.05). The median followup after course 2 was 31.2 months (range 15.6 to 56.4 months). The median interval between courses 1 and 2 of intravesical bacillus Calmette-Guerin was 21.6 months (range 3.6 to 78.8 months). The interval from course 2 of bacillus Calmette-Guerin to progression correlated with the duration of response to course 1 of treatment (p equals 0.01). It appears that a subsequent treatment with bacillus Calmette-Guerin is most likely to be useful in patients who have a sustained response to the initial treatment.