In performing medicolegal autopsies on sudden deaths, there occur a number of cases in which no cause of death can be found. In particular, no evidence of macroscopic cardiac abnormalities can be observed. However, extensive histological screening may reveal isolated areas of myocardial fibrosis. The five cases presented discuss the etiology of this fibrosis and its possible relation to myocarditis. The cases involve white women between the ages of 19 and 25 with no previous medical history. The weight of the heart in all five cases was normal. Macroscopic evidence of fibrosis was visible in four out of five cases. No other macroscopic abnormalities were observed. Histologically, there was evidence of scarring or interstitial fibrosis in all five cases. In four of the cases, additional screening permitted the observation of dispersed inflammatory foci consisting of lymphocytes, plasmocytes and macrophages. Two of the cases demonstrated eosinophil and neutrophil aggregates in the center of necrotic foci. No evidence of vascular inflammatory phenomena was observed in any of the five cases. According to the Dallas criteria, three of the five cases fulfill the requirements for myocarditis and one of the five cases for borderline myocarditis. The Dallas criteria, however, do not take into consideration the possible association between inflammation and myocardial fibrosis since many of the reported series of myocarditis have been from hospital autopsies or endomyocardial biopsies and have not taken into account sudden death from fibrotic sequelae of myocarditis.(ABSTRACT TRUNCATED AT 250 WORDS)