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Extreme alterations in blood count such as anaemia or polycythemia are known to cause circulatory changes and, if these alterations persist, adaptations of cardiac geometry.To investigate further the association between haematocrit levels and left ventricular geometry in a population-based sample.We examined 687 women and 648 men, aged 25–74 years, participating in the third population-based MONICA Augsburg study. Anthropometry, blood pressure, laboratory measurements and M-mode echocardiography were obtained using standardized methods.Haematocrit levels were inversely related to end-diastolic diameters (P < 0.001). By contrast, septal and posterior wall thickness displayed parabolic association curves with nadirs at physiological haematocrit levels (P < 0.001). These associations remained significant after adjustment for age, sex, body fat, hypertension, diabetes mellitus, cardiovascular disease, heart failure, serum creatinine, and were likewise found for haemoglobin levels or numbers of erythrocytes. These correlations appeared to be secondary to changes in blood pressure and stroke volume that correlated either positively (blood pressure) or inversely (stroke volume) with haematocrit levels. Consequently, a concentric pattern of left ventricular hypertrophy, i.e. a relative wall thickness of 0.45 or greater, was significantly more prevalent in subjects with high haematocrit levels than in those with intermediate haematocrit levels. By contrast, an eccentric left ventricular hypertrophy, i.e. relative wall thickness less than 0.45, was more common in subjects with low haematocrit levels.In the general population, the variability of haematocrit levels and its haemodynamic consequences translates to distinct patterns of left ventricular geometry.