A total of 133 patients with cystic fibrosis have been followed for up to 5 years with monthly examinations including bacteriological examinations of sputum. Sera from the patients were examined by means of crossed immunoelectrophoresis for the occurence and number of precipitating antibody specificites against Pseudomonas aeruginosa. Poor prognosis in cystic fibrosis was associated with chronic colonization (9 months - more than 5 years) of the respiratory tract with mucoid Pseudomonas aeruginosa, and with an onset of the chronic colonization before puberty. Among the patients with chronic Pseudomonas aeruginosa colonization, poor prognosis was associated with high numbers of precipitins against antigens from these bacteria (up to 61). The number of Pseudomonas aeruginosa precipitins increased on an average with five per year in chronically colonized patients. Rapidly increasing number of precipitins was associated with poor prognosis. Patients with any degree of impairment of the ventilatory function and any changes on the chest radiographs could contract chronic Pseudomonas aeruginosa colonization. Poor ventilatory function and severe changes on the chest radiographs was associated with high numbers of Pseudomonas aeruginosa precipitins and with poor prognosis. Although many O groups of Pseudomonas aeruginosa were found in the chronically colonized group of patients, 53% of the patients harboured strains belonging to O group 3 or 3/9, and the highest numbers of precipitins were found in serum from these patients.