Nebulised pentamidine was used to treat 30 patients with Pneumocystis carinii pneumonia. Fourteen patients (group 1) received pentamidine isethionate 4 mg/kg (six patients) or 8 mg/kg (eight patients) via a standard jet nebuliser (Acorn, system 22) with a flow rate of 8 l/min. The aerosol droplets had a mass median aerodynamic diameter of 2.6 μm (geometric standard deviation (GSD) 2.9) and 46% of droplets were less than 3.9 μm. A further 16 patients (group 2) received 8 mg/kg pentamidine via a jet nebuliser with baffles to limit droplet size to below 4 μm (Respirgard II). This generated aerosol droplets with a mass median aerodynamic diameter of 0.8 μm (GSD 1.5) and 98% were less than 3.9 μm. Only three of the 14 patients in group 1 responded clinically to treatment, one after the lower dose of pentamidine. Treatment was discontinued in 10 patients and one patient died at bronchoscopy from haemorrhage. Thirteen of the 16 patients in group 2 responded. Side effects occurred infrequently; two patients from group 1 had a cough, six patients (four from group 2) had contact bleeding at bronchoscopy, and two further patients had haemoptysis. The differing response rate may be due to differences in the mean droplet size of the aerosols produced by the nebulisers. Nebulised pentamidine (8 mg/kg) when delivered by Respirgard II nebuliser appears to be as effective as conventional treatment for Pneumocystis carinii pneumonia of mild to moderate severity.