A previous study of tobramycin nebuliser solution (TNS) compared with colistin [C] in cystic fibrosis (CF) patients, chronically infected with pseudomonas, showed benefit for the TNS treated patients over one treatment cycle only. The current report is of an extension of that study. An open randomised cross-over study of TNS compared with C was conducted on 21 patients who had previously been on the 1 cycle study. They continued for a further 5 months and then crossed over to the alternate treatment. There was an advantage for TNS over C in FEV(1) % predicted change over time. The C slope was -0.88% per month and the TNS slope 0.35% per month (p=0.0002). This suggests advantages of TNS over C in a study with a small number of patients. Larger studies are required.