Exercise-training might be a logical method to reverse muscle atrophy and weakness in patients treated with glucocorticoids. The purpose of the present investigation was to establish whether a treatment with low dose prednisone (10 +/- 2.9 mg/d) modulates the effect of a moderate strength type isokinetic training during 7 wk (21 sessions of 20 min) on "muscle efficiency" (power output/muscle mass) and on concomitant changes in ultrastructure of the thigh muscle measured by quantitative electron-microscopic morphometry. Training caused a similar increase in "muscle efficiency" in patients on prednisone (n = 9) as in normal volunteers (n = 9). In normal subjects the increase in muscle efficiency was associated with an increase in sarcoplasm, whereas in patients on prednisone the functional improvement was associated with an increase in sarcoplasm, capillaries, and mitochondria content. Thus, a therapy with low dose prednisone does not abrogate training-induced improvement of muscle efficiency but modulates the ultrastructural response of the muscle to the training.