Aim of this study was to evaluate whether different severity of clinical and functional impairment of right heart in patients with idiopathic dilated cardiomyopathy (IDC) might condition distinct clinical features and prognosis. From 104 consecutive patients with hemodynamically assessed diagnosis of IDC studied between 1977 and 1987 in our Institute, 39 patients (28 males and 11 females, mean age 41 +/- 14 years) were selected on the basis of ejection fraction ranging from 35 to 50%, left ventricular end-diastolic pressure ranging from 13 to 20 mmHg and left ventricular end diastolic volume less than or equal to 150 ml/m2. A significant involvement of right heart (diagnosed according to a mean right atrial pressure greater than or equal to 9 mmHg, a right ventricular end-diastolic pressure greater than or equal to 9 mmHg and a right ventricular end-diastolic diameter greater than or equal to 30 mm) was assessed in 16 patients (41%), 11 males and 5 females, aged 40 +/- 15 years (Group A). On the contrary the remaining 23 patients (59%), 17 males and 6 females, aged 42 +/- 12 years, had a normal right heart (Group B). At entry into the study, clinical features appeared similar in the 2 groups of patients whereas patients of Group A had significantly higher incidence of atrial fibrillation (25% vs 4%, p less than 0.01) and complex ventricular arrhythmia (greater than or equal to 4 Lown class) (25% vs 4%, p less than 0.01) compared with patients of Group B. With respect to conduction defects no difference was found between Group A and Group B.(ABSTRACT TRUNCATED AT 250 WORDS)