Five patients with only mildly dilated ventricles but other features typical of congestive cardiomyopathy underwent cardiac transplantation for class IV NYHA heart failure. The findings of clinical studies, cardiac catheterization, endomyocardial biopsy, and pathologic examination of the removed hearts in this group with mildly dilated congestive cardiomyopathy (MDCM) were compared with similar data in four patients with idiopathic restrictive cardiomyopathy (IRCM) and in 10 patients with typical dilated congestive cardiomyopathy (DCM). In comparison with the other groups, patients with MDCM had a higher incidence of familial cardiomyopathy (p = .02) and a shorter symptomatic period than patients with IRCM (p less than .02). Patients with both MDCM and DCM had globular hearts (with predominant left ventricular dilatation), congestive hemodynamics and poor left ventricular contractility (ejection fraction 12% to 19%), and high incidence of ventricular thrombi. Patients with IRCM showed normal ventricular size, marked atrial dilatation, restrictive hemodynamics, mild-to-moderate decrease in left ventricular contractility (ejection fraction 29% to 55%), and absence of ventricular thrombi. Cardiac index, ventricular wall thickness, and light microscopic findings were similar in the three groups of patients. Electron microscopy showed no myofibrillar loss in patients with IRCM but mild (partial) or moderate-to-severe (almost total) myofibrillar loss in those with MDCM and DCM, respectively. We conclude that end-stage congestive cardiomyopathy may occur without significant ventricular dilatation and patients with MDCM have heart sizes intermediate between those found in IRCM and DCM but their clinical, hemodynamic, and pathologic findings are virtually identical to those of patients with typical DCM.