To investigate whether diaphragmatic strength could be reduced in cystic fibrosis (CF), and to examine possible mechanisms leading to diaphragmatic weakness, we measured transdiaphragmatic pressure (Pdi), together with lung mechanics, including dynamic "intrinsic" positive end-expiratory pressure (PEEPi,dyn), ventilation, lung volumes, and nutritional status in 15 adult patients with CF in stable clinical condition. Diaphragmatic strength was assessed as the maximum Pdi (Pdimax). Nutritional assessment included the calculation of weight as a percentage of ideal weight for height (Wt/Ht). On average, our 15 CF patients had airway obstruction (FEV1 = 59 +/- 28% predicted) and a small PEEPi,dyn (1 +/- 0.7 cm H2O). Functional residual capacity average 52 +/- 9% of the predicted total lung capacity. The Wt/Ht was normal on average (95%), but with a large range from malnutrition to a good nutritional status (76 to 109%). We found that Pdimax decreased with increasing FRC/TLC percent predicted (r = 0.55, p < 0.05), but more significantly with decreasing Wt/Ht (r = 0.76, p < 0.001). The multiple linear regression analysis for these factors was significant (R2 = 0.70, p < 0.05); however, the partial regression coefficient was significant only for Wt/Ht (p < 0.01). These results suggest that in CF patients, diaphragmatic strength decreases with the progression of the disease, increasing lung volume and worsening nutritional status, and that malnutrition is the strongest determinant of diaphragmatic weakness.