Sixty-seven previously healthy patients with acute bronchitis were randomized and treated with either a fixed dose of trimethoprim and sulfamethoxazole or placebo for seven days. All outcomes examined showed a trend favoring the use of antibiotic, with statistically significant differences for cough, night cough, mean temperature, and use of antihistamines or decongestants. Night cough occurred on 84 percent of nights in the control group vs 56 percent in the antibiotic group (P = .003). Cough occurred on 99 percent of days for patients in the control group vs 93 percent of days for patients in the antibiotic group (P = .05). Mean temperature over the seven nights was 37.3 degrees C in the control group vs 36.9 degrees C in the antibiotic group (P = .004). The use of antihistamines and decongestants was reduced from 32 percent of days in the control group to 6 percent of days in the antibiotic group (P = .005). Patients in the antibiotic group worked 73 percent of days vs 55 percent of days for patients in the control group, which was significant when patients were stratified by the appearance of their sputum on Gram stain (P = .006). Smoking history was not found to help predict the response to antibiotic therapy.