In a cross-sectional study we investigated the effect of smoking on hematological parameters in 507 apparently healthy individuals. A positive association was found between smoking and leukocyte count, hemoglobin concentration, packed red cell volume, mean corpuscular volume, and mean corpuscular hemoglobin. By contrast, erythrocyte sedimentation rate was significantly lower in smokers. The number of red blood cells and mean corpuscular hemoglobin concentration were not associated with smoking. When health examinations are carried out, and whenever laboratory studies of healthy or ill persons are evaluated, the effect of smoking on hematological parameters should be considered.