The long-term prognosis of patients with mildly dilated cardiomyopathy (MDCM) was investigated in 21 patients. MDCM was defined as left ventricular ejection fraction ≤40% and left ventricular end-diastolic volume ≤120 ml/m² by left ventriculography. During a follow-up period of 6.8±3.7 years, there were 9 cardiac events (5 heart failure deaths, 2 sudden deaths, and 2 re-hospitalizations for heart failure). Only in the patients without cardiac events was there a significant decrease in left ventricular size (end-diastolic dimension decreased from 58±6 mm to 50±8 mm, p<0.001) and an improvement in systolic function (fractional shortening increased from 17±5% to 26±11%, p=0.007). However, left atrial dilation was observed in the patients with an event (from 39±5 mm to 43±5 mm, p=0.02). Based on proportional hazard analysis, left ventricular end-diastolic pressure and mean pulmonary artery pressure at diagnosis and left atrial dimension at the time of follow-up were significant predictors of poor outcome. A subset of patients with MDCM has impaired hemodynamics at diagnosis, left atrial dilation at follow-up and a poor prognosis, and must be followed carefully even if the left ventricular dilatation is mild. (Circ J 2002; 66: 557 - 560)