We measured static inspiratory and expiratory pressures (PImax and PEmax) in 29 patients with asthma for comparison with a previously reported group of 25 patients with cystic fibrosis (CF) and 80 normal control subjects. The purpose of the study was to assess the relationship between respiratory muscle strength, nutritional status, lung volumes, and training effect of the increased work of breathing in patients with chronic air-flow limitation. PEmax was similar in the asthma group, CF group, and normal control subjects despite a significant degree of malnutrition in the CF group (mean body mass percentile (BMP), 78 versus 107% in the asthma group, p less than 0.001). There was no correlation between BMP and PImax or PEmax in either study group. PImax, when corrected for lung volume, was greater than age- and sex-matched control values in 24% of the patients with CF and 48% of the patients with asthma. We conclude that respiratory muscle strength is not related to nutritional status (as measured by BMP) in patients with CF and asthma. Above-normal inspiratory muscle strength may result from a training effect of the increased work of breathing in both CF and asthma, regardless of nutritional status.