The objective of this study was to quantify the deposition in the lung of a Colistine aerosol generated using a pneumatic nebuliser (Pari LL(R) equipped with a Pari Master, Pari, Germany) and to compare this with the results obtained with an ultrasonic nebuliser (DP100, DP Medical, France) in four subjects suffering from cystic fibrosis being colonised with Pseudomonas aeruginosa. To quantify the pulmonary deposition of the aerosols we have used an indirect isotopic method which consists in assimilating the kinetics of the molecules studied with a serum albumin tagged with Technetium 99m (Tc99mm) and added to a preparation of Colistine. We have previously verified that the addition of a radioactive tracer does not change the normal distribution or dynamics of the medication within the aerosol and the radioactive counter linked to the tracer reflects the mass of the medicament. The pulmonary deposition was expressed as a percentage of the nebuliser dose. A regional analysis of the deposition (central, peripheral, superior and inferior) was carried out and in central deposition compared to the periphery (C/P) and superior compared to inferior (S/I) were calculated. With the DP100 nebuliser the pulmonary deposition of the aerosol was very reproducible from one patient to another, varying only between 9.5 to 14 percent of the nebuliser dose. With the Pari LL the fraction deposited varied more from one patient to another from 5.6 to 27% of the nebuliser dose. In three of four patients, the pulmonary deposition was superior or equal to that obtained with the ultrasonic nebuliser. The patients whose pulmonary deposition was inferior, using the pneumatic nebuliser, was the youngest in the group and co-ordinately poorly the triggering of the nebuliser with the beginning of inspiration. With the two nebulisers, the pulmonary deposition of Colisitine was very heterogeneous throughout the pulmonary parenchyma. The mean of the ratio C/P and S/I obtained in all four patients was identical (1.35 an 0.86 respectively), indicating a deposition of the aerosol which was predominantly central and inferior but was distributed equally in the peripheral parts of the lung. Pneumatic nebulisers offer a reliable alternative notably for domiciliary treatment for Colistine aerosols in patients suffering from cystic fibrosis. In younger patients who have not yet acquired good motor co-ordination, nebulisers which function continuously or are triggered by inspiration seem to be the preferred choice.