Sixty steroid-treated patients with asthma were evaluated for the presence of muscle weakness by use of both manual muscle testing and the Cybex II isokinetic dynamometer. The patients were compared to age and sex-matched sedentary control subjects. Forty-eight percent of the patients (12/25) taking greater than or equal to 40 mg per day of prednisone had hip flexor strength greater than or equal to 2 SD below the mean of age and sex-matched control subjects by Cybex testing (CT). Sixty-four percent of the patients (16/25) taking greater than or equal to 40 mg per day of prednisone were found on manual muscle testing to have hip flexor weakness. Only one patient taking less than 30 mg per day of prednisone was found to have muscle weakness. Biochemical parameters, including CPK, aldolase, SGOT, LDH, and LDH isoenzymes were measured to assess the degree of steroid-induced muscle damage. They neither correlated with the degree of hip flexor weakness as measured by CT, nor did they discriminate between patients receiving small doses and large doses of steroids. Changes in urinary excretion of creatine did not help to confirm the diagnosis of steroid myopathy. Although CT provides an objective means of assessing muscle strength in these patients, at this time no definitive chemical test is available for the diagnosis of steroid myopathy.