Over a 2 years' period, 49 AIDS patients and 3 non AIDS patients were treated for pneumocystosis in our chest department. Forty-six were male and 6 were female. Pneumocystosis was the first opportunistic infection in 77 p 100 of patients. Fever above 38.5 degrees C was the major symptom in 92 p 100. Cough was present in 90 p 100 and dyspnoea in 94 p 100. Clinical symptoms had begun 21.7 +/- 15.7 days before diagnosis. Mean PaO2 value was 50.9 +/- 15.7 mmHg. Forty-eight patients were initially treated by daily intravenous administration of trimethoprim 960 mg and sulfamethoxazole 4,800 mg. Three patients received a pentamidine aerosol and one received DFMO. Treatment was effective in 39 patients; 11 patients died between the 5th and the 29th days of treatment; 2 had an early relapse. Fever disappeared after 9.8 +/- 6.6 days, and blood gases returned to normal within 10.8 +/- 7.7 days. All patients whose PaO2 was above 56 mmHg were cured. Thus, the trimethoprim-sulfamethoxazole combination proved active in the treatment of pneumocystosis. Other treatments are useful in case of side-effects or failure of the initial therapy. Failures can be suspected on the fourth day of treatment and in such cases CMV co-infection must be looked for and treated.