A study was made of 40 patients suffering from clinically and bacteriologically demonstrated bacterial respiratory infections which were all treated with fosfomycin as the only antibiotic. Studies were made of the sensitivity of the isolated organisms and control of the eradicated and selected organisms. Analytical controls were performed on all patients before, during and after treatment. The average dose employed was 60 mg/kg/day, divided into four doses and administered parenterally. The duration of the treatment fluctuated between 6 and 11 days in 87% of the cases. A clinical and bacteriological evaluation was made of the results. The clinical response was satisfactory in 75% of the cases, partial in 7.5% and poor in 17.5%. The bacteriological evolution was satisfactory in 62.5%, partial in 12.5% and poor in 20%. It was not possible to control the evolution in 15% of the patients. The concentrations of the antibiotic were determined in the sputum of 6 patients and fluctuated between 0.6 and 12.9 mug/ml. In no case could any significant disorders be attributed to the antibiotic.