A retrospective study of children dying with active varicella revealed 11 of 17 cases who had unsuspected interstitial myocarditis at the time of their death. In addition, a prospective study of 6 children, consecutively admitted to the hospital with active varicella, were evaluated for evidence of cardiac dysfunction by echocardiography, ECG, and serum enzyme levels. All 6 children had Reye's syndrome in association with active varicella. Evidence of myocardial involvement, consistent with acute congestive cardiomyopathy, was documented in 4 of the 6 children. This study suggests that the heart is commonly involved in varicella infections and that cardiac involvement should be considered in children with this disease.