Using electromyographic techniques, we examined the possibility that idiopathic cardiomyopathy might be a cardiac manifestation of a general myopathy. The EMG studies were done on sixteen normal subjects and seventeen patients with idiopathic cardiomyopathy, including six with hypertrophic obstructive cardiomyopathy (HOCM), six with hypertrophic cardiomyopathy (HCM), and five with congestive cardiomyopathy (CCM). The action potential duration in the brachial biceps were determined exactly by a special recording technique. In the normal subjects, the mean action potential duration was 5.42 +/- 0.12 msec. In the patients with idiopathic cardiomyopathy, the mean action potential duration was 4.43 +/- 0.15 msec, which was significantly shorter than that of the normal subjects; 4.06 +/- 0.30 msec in HOCM, 4.45 +/- 0.20 msec in HCM, and 4.48 +/- 0.13 msec in CCM. These findings suggest that idiopathic cardiomyopathy is only one aspect of a larger disease spectrum, with abnormalities in both voluntary and cardiac muscles.