Antibiotic aerosol treatment is successful in treating Pseudomonas infection in some patients with cystic fibrosis, but the amount of drug reaching the lungs is unknown. The deposition patterns of carbenicillin aerosols delivered from two commercially available nebuliser systems (the Turret nebuliser plus Maxi compressor and the Inspiron nebuliser plus Traveller compressor) have been compared in six patients with cystic fibrosis during tidal breathing. The aerosol mass median diameters were 3.2 and 7.3 microns. In addition, the aerosol from the Turret-Maxi nebuliser system was inhaled by a combination of tidal and deep breathing. After two minutes' breathing via a mouthpiece the mean (SEM) deposition in the lungs was 15.60 (1.5) mg carbenicillin with the Turret nebuliser plus Maxi compressor, but only 6.54 (1.09) mg with the Inspiron nebuliser plus Traveller compressor; the distribution pattern within the lung was significantly more peripheral with the former nebuliser system. These differences may be ascribed partly to the smaller droplet size from the Turret system and partly to the higher nebulisation rate from the more powerful Maxi compressor. Tidal plus deep breathing produced a further small but non-significant increase in lung aerosol deposition. A seventh patient, who failed to complete the trial, had little aerosol deposited in his lungs because he inhaled through his nose. These results emphasise the importance of correct selection of nebuliser equipment for antibiotic aerosol treatment.