During the period 1971-75, 51 cystic fibrosis (CF) patients who contracted chronic P. aeruginosa infection were treated at the Danish CF centre with anti-pseudomonas chemotherapy only when their clinical condition deteriorated considerably. During the period 1976-80, 58 CF patients who contracted chronic P. aeruginosa infection were treated at the Danish CF centre with anti-pseudomonas chemotherapy on a regular basis every 3 months. Each routine 24 day-course of chemotherapy consisted of tobramycin in combination with carbenicillin or other beta-lactam antibiotics with activity against P. aeruginosa. In case of allergy or resistant strains monotherapy with tobramycin was used. The 5-year survival of CF patients from the time of the onset of the chronic P. aeruginosa infection increased from 54% in the first period to 82% in the second period (p less than 0.05), and lung function (peak expiratory flow rate) also improved significantly. It is concluded that intensive "maintenance" chemotherapy against P. aeruginosa improves survival and quality of life of CF patients although permanent eradication of P. aeruginosa is not accomplished.