PURPOSE:To describe the cross-sectional association of counts of total and subtypes of leukocytes, platelets, and selected characteristics of red blood cells with intensity of cigarette smoking and smoking cessation. METHODS:The study population consisted of 16,254 men and women 20-70 years of age who participated in the Dutch European Prospective Investigation into Cancer and nutrition (EPIC) subcohort, 1995-1997. Linear regression analysis was performed, for both sexes separately, to study the relation between various measurements of smoking habits and hematologic characteristics. RESULTS: Among current smokers, in both genders, packyears and daily number of cigarettes were associated with increased leukocytes (6-19%), lymphocytes (4-14%), monocytes (3-9%), neutrophils (7-24%), platelets (1-5%), mean corpuscular volume (0.4-2%), hematocrit (0.3-4%), hemoglobin (0.3-4%), and mean corpuscular hemoglobin (0.8-2%). In subjects who had quitted smoking within two years before blood measurement, hematological characteristics in most cells were similar as in never smokers, whereas normal counts of lymphocytes and monocytes were observed only 2-5 years after quitting. CONCLUSIONS: Our findings suggest a marked influence of current smoking habits on leukocyte counts and, to a lesser extent, on counts of erythrocytes and platelets. Following smoking cessation, at least five years have to pass before changes in all hematological parameters may return to normal values.