Physical training is important in the treatment of patients with cystic fibrosis (CF). Optimal types of training and intensity are unknown. The aim of the study was to evaluate the effect on muscular strength after 6 months of endurance training (ET) and/or resistance training (RT). Twenty patients (eight females) participated, 16-35 years, with mean forced expiratory volume in 1 s 91% of the predicted. ET or RT for 30-45 min three times a week for 3 months was followed by a mixed program for another 3 months. Heart rate recording, diaries and frequent personal contacts were used for monitoring. Vitamin E and cytokines were analyzed. Fifteen tests of muscular strength were used. Handgrip strength in females and quadriceps strength in males were significantly decreased compared with healthy age- and sex-matched controls and positively associated with lung function. Sixteen patients completed the program. By ET, quadriceps strength was further decreased and after 6 months quadriceps isometric strength was also decreased in females. There was a tendency toward different effects on the serum levels of IL-6 and vitamin E by the different types of training. CF patients showed no improvements in muscular strength after 6 months of controlled training, suggesting a physiological muscular impairment despite normal anthropometry, but associated with lung function.