Malnutrition and hyperinflation may both lead to respiratory muscle weakness. To assess separately the effects of chronic hyperinflation and malnutrition on respiratory muscle strength (RMS), 22 subjects with cystic fibrosis (CF) with both hyperinflation and malnutrition were compared to 10 asthmatic patients, a group with hyperinflation without malnutrition, 9 subjects with anorexia nervosa (AN), a group with malnutrition without lung disease, and 14(6 males and 8 females) control subjects with neither compromise. Nutritional status was assessed by body mass percentile (BMP) and percentage ideal weight (PIWT). RMS was diminished in the AN and CF groups (PImax 90 +/- 27, 88 +/- 31 versus 124 +/- 40 cm H2O, p less than 0.05; PEmax 87 +/- 12, 93 +/- 39 versus 121 +/- 32 cm H2O, p less than 0.05), but no difference was found when the AN group was compared with only the female controls. The decrease in PImax in the CF group was primarily due to the mechanical disadvantage placed on the diaphragm by their marked hyperinflation, a mean RV/TLC ratio of 50 +/- 23%. As older CF subjects had previously been shown to have decreased RMS when malnourished, a CF subgroup in the same age range as the controls was evaluated. RMS in this group did not differ from controls despite the presence of malnutrition and hyperinflation. RMS is mildly influenced by nutritional status as assessed by BMP and PIWT but not to any degree of clinical significance.