The hearts of 28 necropsy cases with DCM and 10 control cases were analysed to elucidate the role of myocarditis and fibrosis in dilated cardiomyopathy (DCM). The extent of fibrosis and myocyte diameter were measured in the transverse sections of the left ventricle. The degree and pattern of fibrosis varied widely from case to case. The DCM cases were then classified into 3 groups: 1) Group Ia, 8 cases with mild diffuse fibrosis; 2) Group Ib, 9 cases with severe diffuse fibrosis and 3) Group II, 11 cases with segmental fibrosis. Myocardial fibrosis and hypertrophy were significantly more prominent in Groups Ib and II than in the control group (p less than 0.01), but no differences were observed between Group Ia and the control group. Thus, it is reasonable to attribute the chronic CHF and LV dilatation in Groups Ib and II to the extensive myocardial fibrosis. In contrast, it is not reasonable to attribute the chronic CHF and LV dilatation in Group Ia to myocardial fibrosis. Focal lymphoid cell infiltration was graded to assess inflammatory changes. Group II showed more prominent inflammatory changes than the other 2 groups (p less than 0.05). Thus, it is possible that myocarditis is an etiologic factor for DCM with a segmental pattern of fibrosis.