Six children died of acquired immunodeficiency syndrome (AIDS), four of them females, ages 7 months, 13 months, 2 years 8 months, and 4 years; and two of them males, aged 2 1/2 and 7 years. They were born to IV drug-addicted parents. The conduction system (CS) and the entire heart were studied by serial section. In all cases the heart was hypertrophied and enlarged; one had total thrombotic occlusion of the right coronary artery with extensive infarction of the ventricular septum. Vascular changes also were found in all hearts, involving small arteries, arterioles, and venules. In the arteries, they involved the intima, media, and adventitia, and perivascular areas in a degenerative and inflammatory process. The elastic tissue was especially affected. A nonspecific myocarditis was present in four cases and epicarditis in all. Changes in the summit of the ventricular septum were present in four cases, consisting of increased fibrosis and arteriolosclerosis. The CS changes varied in location, showing either vasculitis, myocarditis, or fragmentation of the bundle with lobulation and fibrosis. The changes in the conduction system were not as severe as the changes in the surrounding myocardium. In one case the ECG was abnormal, showing left hemiblock. This corresponded to the finding of fibrosis, vacuolization of cells, and space formation in the left bundle branch.