The effectiveness of biweekly administration of 60 mg aerosolised pentamidine (AP) as secondary prophylaxis against relapse of Pneumocystis carinii pneumonia (PCP) was investigated in 45 male AIDS patients. The nebuliser used was an Acorn System 22. In total the patients received AP for a mean period of 13.8 months (3.4-28.8). Six episodes of recurrent PCP were recorded. Relapse per full year of treatment on AP was 12%.