To create a model for studying the regional variation in the histologic structure of left ventricular myocardium and the representativity of samples from endomyocardium in predicting the state of myocardium as a whole, 12 human hearts (3 normal and 9 pathological) were investigated with histoquantitative methods. Ten small endomyocardial areas representing biopsy sites (mean area on section 0.94 mm<sup>2</sup>) were delineated from each heart. The volume fractions of myocardial fibers (V<sub>VMY</sub>), interstitial space (V<sub>VINT</sub>) and connective tissue (V<sub>VCT</sub>) were determined with a point counting method on these areas. Also the mean fiber diameter was measured. The results were compared with the results from three full thickness samples (mean area 3 cm<sup>2</sup>) from each heart. The values for fiber diameter estimates were almost identical in endomyocardial samples and full thickness samples. On the other hand, V<sub>VCT</sub> was significantly higher and V<sub>VMY</sub> lower in endomyocardial samples. The coefficient of variation (10 endomyocardial areas) was 8.3% for fiber diameter, 17.8% for Vvmy and 77.5 % for Vvct· When estimates of the regional variation within the heart area are available, it is possible to create a model for the prediction of the number of endomyocardial biopsies needed to reach a given accuracy and probability of measurements. The representativity and clinical value of endomyocardial biopsies are discussed on the basis of this statistical approach.