We examined the association of natural killer (NK) cell activity with a number of life style factors by a cross-sectional analysis on 2892 Japanese individuals. The following habits were found to be associated with increased NK activity: 1) alcohol drinking (males, P less than 0.05), 2) not smoking cigarettes (males and females, P less than 0.001), 3) increased intake of green vegetables (females, P less than 0.001), 4) increased intakes of meat, milk, dairy products, and soybean products (males, P less than 0.01), 5) daily workload of less than 3 hours per day (females, P less than 0.05), 6) regular meals (females, P less than 0.05), 7) regular sleep of more than 7 hours (females, P less than 0.05), 8) proper body weight. In addition, systolic blood pressure showed a positive correlation with NK activity (P less than 0.001), while proportion of helper/inducer (OKT4+) T cells and fraction of beta-globulin showed negative correlations (P less than 0.001). We thus found that the living habits associated with increased NK activity were consistent with generally accepted good health practices, except alcohol drinking.