The objective of this study was to assess the prevalence of covert congenital cardiac malformations. A retrospective analysis was made of a personal series of 4,953 consecutive postmortem examinations performed over 35 years (1946-1980) in a mixed urban and rural region of North Lincolnshire, England. The subjects were 4,210 adults and 743 children (under the age of 16 years). There were 1,635 hospital cases and 3,318 coroner's cases; of the latter, 2,651 were instances of sudden and unexpected death. Four adults and 99 children with known or suspected congenital heart disease were excluded from the analysis. Of the remaining 4,850 cases, 25 had benign covert cardiovascular malformation (0.52%), and in approximately 54 more a covert malformation was considered to have been a possible contributor to death. This latter group consisted of 44 cases of bifoliate calcified aortic valves, 6 cases of cardiomyopathy, and 4 miscellaneous cases. As sudden deaths were grossly over-represented in the series, an approximate correction factor reduced the prevalence of this group to 0.16%, giving a prevalence of 0.68% for all covert cases. According to these data, the true prevalence of congenital cardiac malformations is probably about twice that of the generally accepted level in live births. The implications for epidemiological studies pertinent to the etiology of congenital heart disease are indicated.