The suppression of pathogenic aerobes and the preservation of anaerobes provides a degree of infection prevention during granulocytopenia. Trimethoprim/sulfamethoxazole (TMP/SMZ) suppresses Enterobacteriaceae and probably maintains colonization resistance through sparing of anaerobes. TMP/SMZ (320/1600 mg/day) treatment was compared to placebo in a double-blind, randomized trial in patients with newly diagnosed small cell carcinoma of the lung during the initial courses of chemotherapy with cyclophosphamide, doxorubicin, and etoposide. Infections were evaluated as microbiologically documented, with or without bacteremia, and clinically documented and were correlated to granulocytopenia. Of the 61 patients evaluated, 32 were given TMP/SMZ and 29 were given placebo; both groups had similar characteristics with regard to disease extent, performance status, age, sex, chemotherapy, and days of granulocytopenia. Incidence of infection at less than 100 granulocytes/microliters was significantly reduced in the TMP/SMZ group (2 infections/100 days) compared to placebo (11 infections/100 days, p = 0.005). Also reduced were the number of bacteremias and the mean proportion of study time on broad-spectrum antibiotics (p less than 0.01). Compared to placebo, TMP/SMZ provided infection prophylaxis without an increase in marrow suppression among patients with small cell carcinoma of the lung receiving intensive chemotherapy.