The purpose of this study was to estimate the frequency of and evaluate the clinical impact of pulmonary mycobacterial infections among cystic fibrosis (CF) patients. 185 CF patients aged 2.2-38.5 years were screened by sputum samples and by intracutaneous skin tests against tuberculin and sensitins produced from Mycobacterium chelonae subsp. abscessus, M. avium, M. intracellulare and M. scrofulaceum (the MAIS complex). The skin tests towards the sensitins in BCG-vaccinated patients (n = 60) were significantly influenced by the vaccination. 26 of the remaining 125 non-vaccinated patients had > or = 1 positive skin test (95% confidence limits 15-29%). The majority reacted against the MAIS complex. However, the reactions were similar to those of healthy siblings and an age-matched control group. Moreover, the lung function, growth and HbA1c were similar among skin test positive and negative patients. Three patients had repeated positive sputum cultures, the point prevalence being 1.6% (M. intracellulare, n = 2 and M. chelonae subsp. abscessus, n = 1). During the subsequent 4 years, 4 additional patients with M. chelonae subsp. abscessus were identified. Based on clinical observations, 5 of the infected patients were considered asymptomatic, while 2 might have been symptomatic. In 1 patient, M. chelonae subsp. abscessus disappeared spontaneously. Despite intensive treatment with new antibiotics against Mycobacteria Other Than Tuberculosis (MOTT) in 4 patients, the mycobacteria were not eradicated. In conclusion, MOTT infection was rare and the clinical impact difficult to prove. Treatment should focus on clinical improvement in the individual patient suspected of suffering from significant symptomatic infection. Eradication of the bacteria should not be expected.