Major lymphocyte subsets were quantitated using immunofluorescence staining and flow cytometric analysis in Korea Air Force pilots undergoing continuous training and intensive situation both physically and psychologically. Total 197 subjects were included in this study. Young students assayed in parallel as group I (28 subjects) control group. The pilots were divided into two groups, group II 2nd and 1st lieutenants and group III(93) captain and major, according to the rank. The percentage of CD4 helper T lymphocyes was higher in group III (48.91+/-10.11%) Than group I (45.42+/-4.73%) or II(47.08+/-8.28). The percent of CD8 cytotoxic T lymphocytes of group III (23.53 5.64%) was lower than that of group I (28.91+/-3.37) or group II (25.95+/-6.36). Consequently, CD4/CD8 ratio was higher in group III (2.24+/-0.79) than in group I(1.59 +/-0.27) or group ~l.95+/-0.66). Total white blood count of group III (6969+/-1335 count/mm2) was lower In group III than that of groupI (7639+/-1788) or group II(7210+/-1653), but no statistical differences were noted in lymphocyte count and Percents of CD16 natural killer cells and CD19 B lymphocytes. In light of the functional significance of the lymphocyte subsets Investigated, It is suggested that the present stress-induced alterations may have practical implications.