We measured maximal inspiratory and expiratory pressures (MIP and MEP, respectively) in 23 male patients with cystic fibrosis (CF), 16 to 35 yr of age (22.1 +/- 3.7), and in a control group of 33 male volunteers, 17 to 39 yr of age (22.5 +/- 6.8), to evaluate the effects of chronic hyperinflation and malnutrition on MIP and MEP in the patients with CF. Routine pulmonary function tests and skeletal muscle indexes, such as the force generated by the adductor pollicis muscle with supramaximal ulnar nerve stimulation at a frequency of 10 Hz as percentage of force at 100 Hz (F10/100) and midarm muscle circumference as a percentage of predicted (MAMC), were also measured in the patients with CF. Severe hyperinflation in this study was defined as a ratio of residual volume to total lung capacity above 50% and malnutrition as a ratio of actual weight to the ideal weight for the patient's age and height of 90% or less. The severely hyperinflated subgroup of patients with CF had significantly reduced MIP values in comparison with those in the other patients with CF. The malnourished subgroup, which was also severely hyperinflated, differed from the well-nourished one in both skeletal muscle indexes; MAMC was reduced, whereas F10/100 was elevated, and respiratory muscle pressure generation, MIP, and MEP were reduced. We conclude that patients with CF who are malnourished and/or severely hyperinflated have reduced maximal respiratory pressures.