Abstract Little information is available about the effects of inspiratory muscle training in patients with cystic fibrosis (CF). In this study the effects of inspiratory-threshold loading in patients with CF on strength and endurance of the inspiratory muscles, pulmonary function, exercise capacity, dyspnoea, and fatigue were evaluated. Sixteen patients were assigned to one of two groups using the minimization method: eight patients in the training group and eight patients in the control group.The training was performed using an inspiratory-threshold loading device. Patients were instructed to use the threshold trainer 20 min a day, 5 days a week for 6 weeks. Patients in the training group trained at inspiratory threshold loads up to 40% of maximal static inspiratory pressure (Pimax) and patients in the control group got ‘sham’ training at a load of 10% of Pimax. No significant differences were found among the two groups in gender, age, weight, height, pulmonary function, exercise capacity, inspiratory-muscle strength and inspiratory-muscle endurance before starting the training programme. Mean ( SD ) age in the control group was 19 (5·5) years, mean ( SD ) age in the training group was 17 (5·2) years. Mean FEV 1in both groups was 70% predicted, mean inspiratory-muscle strength in both groups was above 100% predicted. All patients except one, assigned to the training group, completed the programme. After 6 weeks of training, mean inspiratory-muscle endurance (% Pimax) in the control group increased from 50% to 54% ( P=0·197); in the training group mean inspiratory muscle endurance (% Pimax) increased from 49% to 66% ( P=0·003). Statistical analysis showed that the change in inspiratory-muscle endurance (% Pimax) in the training group was significantly higher than in the control group ( P=0·012). After training, in the training group there was a tendency of improvement in Pimax with an increase from 105 to 123% predicted, which just fell short of statistical significance ( P=0·064). After training no significant differences were found in changes from baseline in pulmonary function, exercise capacity, dyspnoea and fatigue. It is concluded that low-intensity inspiratory-threshold loading at 40% of Pimax was sufficient to elicit an increased inspiratory-muscle endurance in patients with CF.