To obtain epidemiologic information on extra echocardiographic spaces immediately posterior to the left ventricular free wall, 2,028 subjects in the original Framingham cohort study (mean age 70 +/- 7 years) and 3,624 of the offspring of the cohort (and their spouses) (mean age 44 +/- 10 years) with adequate echocardiograms were evaluated. Extra echocardiographic spaces were detected in 370 (6.5%) of the 5,652 subjects. The prevalence ranged from less than 1% in subjects in the 20- to 30-year age decade to greater than 15% for those in their 80s. Extra echocardiographic spaces tended to be more common in subjects who were older, female, obese, more hypertensive, and who had higher blood sugar levels and higher low density lipoprotein cholesterol levels (measured 8 years earlier). The high prevalence of extra echocardiographic spaces and the independent association with age (cohort and offspring), obesity (cohort and male offspring), and ventricular septal hypertrophy (cohort and male offspring) is compatible with at least 2 hypotheses among others that should be tested: (1) Subepicardial fat may often masquerade as pericardial fluid producing a posterior extra echocardiographic space, especially in obese elderly subjects. (2) Small posterior extra echocardiographic spaces may often be early markers of subclinical hypertensive heart disease.