Estimates of the incidence of idiopathic dilated cardiomyopathy (IDC) were obtained by identifying cases from Washington County, Maryland who were hospitalized at local and regional hospitals over a recent 17 year period. All of the verified cases had evidence of ventricular dilatation and hypokinesis, with a left ventricular ejection fraction of less than 40%. A total of 26 cases were hospitalized over the period 1975-1991. The average age of the cases at the time of diagnosis was 55.5 years (range 22-80 years of age). About 58% of the subjects were males. A marked increase in newly diagnosed cases of IDC occurred in this population in recent years; 38% of the cases (10 of 26) were newly diagnosed during 1991 alone. Over the period 1975-1991, the average annual incidence rate of hospitalized cases was 1.6 per 100,000 among men and 1.2 per 100,000 among women. During the last 7 years of the case ascertainment period (1985-1991), the average annual incidence rate was 3.5 per 100,000 among men and 2.5 per 100,000 among women. These results provide additional evidence that IDC is being increasingly recognized by clinicians, and underscore the need for an improved understanding of the distribution and determinants of this often fatal condition.