Secondary hypertrophic cardiomyopathy (HCM) was diagnosed in 55 newborns and infants without heart disease. The 17 newborn infants of diabetic mothers, 23 infants given ACTH treatment for infantile spasms, 9 ventilated premature infants or babies with bronchopulmonary dysplasia and 6 infants with different underlying diseases comprising this group underwent 2-dimensional echocardiography. In almost all cases echocardiographic evidence of HCM resolved after removal of the exogenous or endogenous causative factor. Myocardial effects of several substances are known and their pathophysiological mechanisms are discussed. Since HCM may precede systemic disease or may often be the first pointer to a triggering agent, further diagnostic procedures are always indicated. Hence, the diagnosis of primary HCM should be made by exclusion.