To test whether early treatment could postpone the chronic colonisation of the respiratory tract with mucoid strains of Pseudomonas aeruginosa in patients with cystic fibrosis, we performed a pilot study in 28 patients aged 2 to 18 years. A two week course of azlocillin (150 mg/kg/day) and tobramycin (10 to 15 mg/kg/day) was given after a mean duration of P aeruginosa colonisation of five months (range one to 11 months). Weight for height increased significantly by 3.5% (SEM 0.7%) of the predicted normal after chemotherapy. The eradication of P aeruginosa that was achieved in 18 children directly after hospital treatment was only temporary. Samples from only 10 and five patients remained negative three and six months after treatment, respectively. Five children remained free from P aeruginosa for a prolonged period of 14 to 32 months. We conclude that, apart from the clinical improvement in all patients, some children might benefit from early antipseudomonas treatment with respect to the bacteriological outcome. Most children, however, experience only a temporary reduction in colonisation. Further investigations in form of controlled clinical trials seem justified.