The course of coxsackievirus B4 necrotizing myocarditis was studied over a 12-month period in 712 ICR Swiss mice inoculated at less than 48 h of age. Affected animals were sacrificed at intervals until 1 year. Microscopically, focal myocardial necrosis, which was often transmural in extent with mixed inflammatory exudate, and subsequent fibrous replacement were induced in 75% of the examined animals. The left ventricle (63 of 69 subjects, 91.3%), interventricular septum (39 of 69 subjects, 56.5%) and right ventricle (26 of 69 subjects, 37.7%) were most frequently involved. Thinning of the ventricular wall and grossly apparent localized ventricular bulges (aneurysms) were seen in 22 subjects (the left ventricle 16 times, the right ventricle 5 times, and the interventricular septum 5 times). Coronary arteries were normal in all instances.