The total white blood cell (WBC) count is reported to be an independent predictor of mortality in several prospective studies. We investigated the association between total and differential WBC counts and cigarette smoking habit in a cross-sectional population-based study of 6902 men and 8405 women 39-79 years of age participating between July 1994 and 1997 in the European Prospective Investigation of Cancer (EPIC-Norfolk) study. Main outcome measures included WBC, granulocyte, lymphocyte and monocyte counts measured at a baseline health check and self-reported cigarette smoking habit. The age- and body mass index-adjusted mean total WBC counts were 7.8, 6.4, and 6.2x10(3) per ul (P<0.0001) among male current, former and never smokers, respectively, and 7.4, 6.3 and 6.2x10(3) per ul (P<0.0001), respectively, in women. The greatest absolute and percentage differences between smoking groups were observed for the granulocyte count. Current smoking habit had a stronger effect on mean total WBC counts than cumulative exposure as measured by pack years. Among former smokers mean age- and body mass index-adjusted WBC, granulocyte and lymphocyte counts were inversely related to duration of smoking cessation (P< or =0.02). Smokers who had given up less than 12 months previously had WBC counts substantially lower (6.7 and 6.9x10(3) per ul, respectively, in men and women) than current smokers. In conclusion, the total WBC count and its components (particularly the granulocyte count) are strongly associated with cigarette smoking habit. Smoking cessation may have an almost immediate impact at least on pathophysiologic processes such as inflammation that may be indicated by the WBC count. The apparent almost immediate reversibility of effects of smoking on inflammation, as indicated by the WBC count, may help motivate efforts to stop smoking.