A combination of sulfamethoxazole and trimethoprim (Bactrim) was given orally to 35 cancer patients with infections. Thirty-two patients did not respond to an initial antibiotic regimen that consisted primarily of carbenicillin disodium and an aminoglycoside. There were 18 single-organism, Gramnegative infections. The overall cure rate was 54%. The most common infection was pneumonia (47% responded to treatment). Eighty percent of the cases of septicemia were cured. The most common infecting organism wasKlebsiella pneumoniae(45% with this infection responded). Eight cases of infection of unknown origin occurred (63% responded to treatment). Overall, 47% of the patients whose neutrophil count remained unchanged or decreased responded, while 61% of those whose neutrophil count remained unchanged or increased responded. There was no close correlation between the minimum inhibitory concentrations and the clinical responses. Sulfamethoxazole-trimethoprim orally is a well tolerated and effective form of antimicrobial therapy. (JAMA237:352-354, 1977)