The authors conducted a 10-year prospective cohort study of mortality in relation to white blood cell counts of 437,454 Koreans, aged 40-95 years, who received health insurance from the National Health Insurance Corporation and were medically evaluated in 1993 or 1995, with white blood cell measurement. The main outcome measures were mortality from all causes, all cancers, and all atherosclerotic cardiovascular diseases (ASCVD). Hazard ratios and 95% confidence intervals were calculated using Cox proportional hazards models with adjustment for age and potential confounders. During follow-up, 48,757 deaths occurred, with 15,507 deaths from cancer and 11,676 from ASCVD. For men and women, white blood cell count was associated with all-cause mortality and ASCVD mortality but not with cancer mortality. In healthy nonsmokers, a graded association between a higher white blood cell count and a higher risk of ASCVD was observed in men (highest vs. lowest quintile: hazard ratio = 2.10, 95% confidence interval: 1.50, 2.94) and in women (hazard ratio = 1.35, 95% confidence interval: 1.17, 1.56). In healthy smokers, a graded association between a higher white blood cell count and a higher risk of ASCVD was also observed in men (highest vs. lowest quintile: hazard ratio = 1.46, 95% confidence interval: 1.25, 1.72). These findings indicate that the white blood cell count is an independent risk factor for all-cause mortality and for ASCVD mortality.