Abstract: A pharmaceutical composition comprising an aqueous solution of pentamidine or a pharmaceutically acceptable salt thereof, the solution having a pH of less than 4.5. The pH of the solution is preferably less than 4.0, more preferably less than 3.5, and especially less than 3.0.Also disclosed is a method for the prophylactic or remedial treatment of pneumo-cystis carinii pneumonia, which method comprises administration by inhalation to a patient having or being susceptible to that condition of a therapeutically effective quantity of an aqueous solution of pentamidine, or a pharmaceutically acceptable salt thereof, the solution having a pH of less than 4.5.