Pentamidine concentrations were determined in plasma after a single aerosolization of 4 mg/kg pentamidine base on 18 patients breathing spontaneously (Group I) and in eight patients receiving mechanical ventilation (Group II). All the patients had documented pneumocystosis. Large interindividual variations in concentrations appeared, especially in Group I. Low concentrations were observed in Group I: Cmax = 65.6 +/- 9.4 micrograms/L (mean +/- SEM), contrasting with high levels in Group II: Cmax = 215.8 +/- 49.8 micrograms/L (mean +/- SEM). Consequently, the mean area under the curve from zero to 4 h was 2.6-fold higher in Group II than in Group I. These findings underline the risk of dose-related pentamidine toxicity in ventilated patients treated with aerosolized pentamidine and the interest of plasma pentamidine monitoring.