We studied the conduction system of 65 cases of proven active or healed myocarditis and related diseases among 7120 autopsy samples. For this purpose, we prepared serial sections by Lev's method. The pathological diagnoses were idiopathic acute myocarditis (5), giant cell myocarditis (3), chronic myocarditis (13), healed myocarditis (22), sarcoidosis (4), collagen or autoimmune disease (13) and complication of cachexia (5). Among all the autopsy cases, Fiedler's myocarditis was found in only one case, but myocarditis was revealed in 19 out of 30 cases of dilated cardiomyopathy, and 15 out of 25 cases of sick sinus syndrome. Conduction system lesions were divided into two groups. In older cases manifesting mainly arrhythmia, the SA node, atrial muscle and AV node were involved concomitantly with perimyocarditis. In younger cases mainly showing heart failure, the RBB, LBB and Purkinje fibers were damaged by endomyocarditis. Histologically, interstitial myocarditis was observed in the former group and parenchymatous myocarditis in the latter.