To identify the evidence of presymptomatic manifestations of dilated cardiomyopathy (DCM), we studied 30 patients with latent DCM (mean 37 +/- 14 years) who satisfied the following criteria: 1) left ventricular (LV) systolic function was slightly reduced; 2) LV end-diastolic dimension (< 54 mm) and coronary arteries were normal. The incidence of ECG abnormalities was relatively high; nonspecific ST-T changes were the most common (90%). The incidence of grade 3 or 4 ventricular premature contractions according to Lown's classification on 24 hour ambulatory ECGs was 50%. Perfusion defects were observed in 83% on the thallium-201 images. Right ventricular biopsy showed that the interstitial fibrosis was milder (9 +/- 9%) in the study subjects than in 32 patients with DCM (17 +/- 8%) who were treated in our hospital, but there were no significant differences in the diameters of the myofibers (15 +/- 4 vs 17 +/- 4 microns) between the 2 groups. During the follow-up study, deterioration of LV dysfunction was observed in 3 patients. One patient died suddenly. These findings proved the importance of the early detection and characterization of latent state of DCM.