There are sex differences in many inflammatory and immune diseases, and the differences tend to diminish after menopause. The underlying reasons are unclear, but sex hormone levels are likely to be an important factor. Blood leukocyte count and composition provide an indicator of the inflammatory and immune status of an individual. We performed a cross-sectional analysis of blood leukocyte data from 46,879 individuals (26,212 men and 20,667 women, aged 18 to 93 years) who underwent a routine health checkup. In women aged around 50 years, neutrophil percentage (NE%) dropped whilst lymphocyte percentage (LY%) rose. Accordingly, women before age 50 had significantly higher NE%, lower LY%, and higher neutrophil-to-lymphocyte ratio (NLR) than women of 51-70 years of age (p = 1.35×10-82, p = 5.32×10-100, and p = 1.25×10-26, respectively). In age groups of <50 years, women had higher NE%, lower LY% and higher NLR than men (p = 1.82×10-206, p = 1.46×10-69, and p = 2.30×10-118, respectively), whereas in age groups of >51 years, it was the reverse (p = 1.92×10-15, p = 1.43×10-84, and p = 1.51×10-48, respectively). These results show that blood leukocyte composition differs between women before and after menopausal age, with distinct sexual dimorphism.