BACKGROUND To investigate the association between lifestyle factors and immune responses to tumors, we determined lymphokine-activated killer (LAK) cell activity and proportions of lymphocyte subsets in 54 healthy males. METHODS LAK cell activity was determined by 51Cr release assay. The proportions of natural killer cells (CD3-16+56-, CD3-16-56+ or CD3-16+56+ cells), helper/ inducer T cells (CD4+8-), and cytotoxic/suppressor T cells (CD4-8+) in lymphocytes were also determined by flow cytometry. Subjects were classified into two groups, good or poor overall lifestyle, according to their answers on a questionnaire regarding eight health practices (cigarette smoking, alcohol consumption, physical exercise, mental stress, sleeping pattern, nutritional balance, eating breakfast, working pattern). RESULTS Subjects with good health practices regarding smoking showed significantly higher LAK cell activity. They also showed a significantly higher proportion of natural killer cells and a lower proportion of helper/inducer T cells. LAK cells activity was significantly higher in subjects with a good overall lifestyle than in those with a poor overall lifestyle after controlling for the effects of smoking. There were no significant differences in the proportions of lymphocyte subsets between the two lifestyle groups. CONCLUSIONS Lifestyle factors are associated with LAK cell activity and the proportion of lymphocyte subsets.