Twenty-one patients with chronic obstructive pulmonary disease (COPD) or asthma, admitted to our division because of exacerbation of their conditions and requiring intensified treatment with corticosteroids, underwent pulmonary function tests, tests of respiratory muscle function, measurement of quadricep strength, and a variety of anthropometric and biochemical measurements. All tests were performed the 10th day after admission. As expected, muscle strength and pulmonary function were interrelated. Surprisingly, the average daily dose of steroids taken in the previous 6 mo, which ranged from 1.4 to 21.3 mg (average 4.3 mg), was significantly related to inspiratory muscle strength (PImax) and a similar tendency was present for expiratory muscle strength (PEmax). Multiple regression analysis of the relationship between PImax and quadriceps force (QF) and steroid dose revealed that the average daily dose independently explained 32% of the variance in PImax and up to 51% of the variance in QF. These relationships were independent of the degree of bronchial obstruction estimated by percentage predicted FEV1. Other significant determinants were age, sex, and COPD for PImax and age, sex, and body weight for QF. The present study demonstrates that in patients with COPD or asthma, respiratory and peripheral muscle strength and steroid treatment are interrelated despite the relatively low doses administered. This observation imposes further limitations on the prolonged treatment of chronic airflow obstruction with systemic corticosteroids.