The aim of this multicentric randomised controlled trial was to evaluate long-term efficacy and safety of once-monthly versus twice-monthly 300 mg aerosolized pentamidine (AP) as secondary prophylaxis of Pneumocystis carinii pneumonia (PCP). We randomised 205 patients with a previous confirmed episode of PCP (107 treated with 300 mg once-monthly AP, and 98 with 300 mg twice monthly AP); the median review period was 232 days. Kaplan-Meier method and Cox's hazard regression model were used for analysis. The main outcome assessments were PCP recurrence, survival and incidence of drug toxicity. The two groups were balanced for prognostic predictors. In the once-monthly AP group, 14 relapses of confirmed PCP were observed, while five occurred in the twice-monthly AP group; the crude relative risk (RR) was 2.69 (95% CI 1.002-7.236, P=0.0496) and the adjusted RR accounting for prognostic predictors was 2.62 (95% CI 0.92-7.5, P=0.071). Death occurred in 36 of 26 patients respectively (adjusted RR 1.32, 95% CI 0.8-2.18, P=0.28). Two patients interrupted the study because of intolerance to AP (one in each group), and severe coughing occurred in two patients (one in each group). At the end of the study, pulmonary function tests were not changed compared with baseline and were the same between the two groups. Our study suggests that 300 mg twice-monthly AP is more effective than 300 mg once-monthly AP as secondary prophylaxis of PCP.