Although idiopathic cardiomyopathies are prognostically important and are a common indication for cardiac transplantation in all age groups, the incidence and age distribution of idiopathic cardiomyopathies in a well-defined pediatric population have been poorly characterized. A retrospective study was carried out in Finland in 1980-1991 to obtain information on the epidemiology of childhood cardiomyopathies. The medical records of all patients aged birth to 20 years with cardiomyopathy from the five university hospitals and 16 central hospitals covering the entire country were reviewed. Moreover, data on causes of death from the Finnish National Census Bureau were examined. Of the 808 potential cases screened, 118 infants, children, and adolescents, representing an average age-specific population of 1.4 million, were definitely identified as having idiopathic cardiomyopathy. The average annual occurrence of new cases was 0.65 per 100,000 population (95% confidence interval (CI) 0.53-0.79). If the 15 cases diagnosed only after death during the 12-year study period were included, the occurrence increased to 0.74 per 100,000 population per year. Fifty-six new cases of dilated cardiomyopathy and 40 new cases of hypertrophic cardiomyopathy were diagnosed during the study period, giving average annual occurrences of 0.34/100,000/year (95% CI 0.26-0.44) and 0.24/100,000/year (95% CI 0.17-0.33) for new cases of dilated and hypertrophic cardiomyopathies, respectively. At the end of 1991, the prevalence of dilated cardiomyopathy was 2.6/100,000 (95% CI 1.8-3.6) and that for hypertrophic cardiomyopathy was 2.9/100,000 (95% CI 2.0-4.0). The number of new cases of dilated cardiomyopathy per year increased over the study period, whereas the annual occurrence of hypertrophic cardiomyopathy remained relatively constant. Marked variability was seen in occurrence among the different age groups of children with dilated cardiomyopathy, suggesting that different pathophysiologic mechanisms, and possibly etiologies, may exist in different age groups.