Our series of 195 patients, plus 134 reported on in the literature and 949 reviewed by various physicians provide 1,278 patients for review of bacillus Calmette-Guerin therapy complications. Cystitis occurred in 91 per cent of the patients. Complications identified included fever more than 103F in 50 patients (3.9 per cent), granulomatous prostatitis in 17 (1.3 per cent), bacillus Calmette-Guerin pneumonitis or hepatitis in 12 (0.9 per cent), arthritis or arthralgia in 6 (0.5 per cent), hematuria requiring catheterization or transfusion in 6 (0.5 per cent), skin rash in 5 (0.4 per cent), skin abscess in 5 (0.4 per cent), ureteral obstruction in 4 (0.3 per cent), epididymo-orchitis in 2 (0.2 per cent), bladder contracture in 2 (0.2 per cent), hypotension in 1 (0.1 per cent) and cytopenia in 1 (0.1 per cent). Most of the severe irritative side effects and subsequent systemic complications can be prevented with prophylactic isoniazid given for 3 days, beginning the morning of treatment. Patients with life-threatening systemic bacillus Calmette-Guerin infection or anaphylaxis should receive 500 mg. cycloserine twice daily for 3 days in addition to combination antituberculous therapy because the rapid action of this drug may be life-saving. Direct intralesional bacillus Calmette-Guerin immunotherapy can produce sepsis and death, and should be avoided but intravesical bacillus Calmette-Guerin generally is well tolerated and has produced no complication in more than 95 per cent of the patients treated.