Increased heart weight is known to be associated with polyploidy of myocardial nuclei. However, the relation between polyploidy and myocardial hypertrophy at a cellular level, ie myocyte hypertrophy, has not yet been investigated. The present study was conducted to reveal this relation in different regions in the heart using two recently developed techniques, ie flow cytometry to measure polyploidy and microscopic morphometry to measure myocyte dimensions in fixed tissue sections. We investigated the effect of pressure overload induced hypertrophy in subendocardial and subepicardial regions of 43 human left ventricles and transmural samples from 47 human right ventricles. In all regions the degree of polyploidy correlated well with myocyte volume indexed for body surface area. At equal extent of myocyte hypertrophy, the degree of polyploidy was higher in left ventricular samples than in right ventricular samples. In conclusion, pressure overload induced myocyte hypertrophy is associated with polyploidy, which is more pronounced in the left ventricle than in the right ventricle.