A restrictive hemodynamic profile with left ventricular (LV) end-diastolic volume <100 ml/m² and LV enddiastolic pressure > 18 mm Hg, in the absence of endomyocardial, pericardial, and specific cardiomyopathy, is a peculiar feature of primary restrictive cardiomyopathy. From 1985 to 1994, 7 hearts of patients who met the above hemodynamic criteria and underwent endomyocardial biopsy because of heart failure, were studied through gross (5 cardiectomies and 2 autopsies), histologic, and electron microscopic investigations. Ages ranged from 9 to 48 years (mean age 29 ± 13). Four patients (57%) had a positive family history: 2 for hypertrophic and 2 for restrictive cardiomyopathy. Three patterns were identified in the 7 hearts: (1) pure restrictive form in 4 cases with mass/volume ratio 1.2 ± 0.5 g/ml, ejection fraction 58 ± 5%, LV end-diastolic volume 67.5 ± 12.6 ml/m², LV end-diastolic pressure 26.7 ± 3.5 mm Hg; (2) hypertrophic-restrictive form in 2 cases with mass/volume ratio 1.5 ± 0.07 g/ml, ejection fraction 62 ± 1%, LV end-diastolic volume 69 ± 10 ml/m², LV end-diastolic pressure 30 ± 7 mm Hg; and (3) mildly dilated restrictive form in 1 case with mass/volume ratio 0.9 g/ml, ejection fraction 25%, LV end-diastolic volume 98 ml/m², LV end-diastolic pressure 40 mm Hg. Histology and electron microscopy disclosed myocardial and myofibrillar disarray and endoperimysial interstitial fibrosis in each pattern. The familial forms suggest the presence of a genetic abnormality. Primary restrictive cardiomyopathy may present with or without hypertrophy and shares similar microscopic pictures with hypertrophic cardiomyopathy. The 2 entities may represent a different phenotypic expression of the same genetic disease.