The cardiac effects of dermatomyositis and polymyositis are found frequently at autopsy, even though the clinical manifestations are rare. We report the observation of a patient with dermatomyositis, in whom ventricular tachycardia, and dilated hypokinetic cardiomyopathy were in the foreground of the cardiac anomalies. This rhythm disturbance responds to treatment, as opposed to the associated dilated cardiomyopathy. Electrocardiographic anomalies are frequent, notably conduction disturbances. Ventricular rhythm disturbances are of poorly defined frequency. Cardiac insufficiency is often stabilised with treatment. The other effects are much rare.