This study examines the levels of carbohydrate antigen 19-9(CA19-9) and cancer antigen 125(CA125) in serum and its related factors in healthy Korean population. Although CA19-9 and CA125 have been widely used tumor markers for gastroenteric cancers and ovarian cancer in Western countries, there are no information available on the serum levels of CA19-9 and CA125 in healthy population and the factors affecting the levels of these tumor markers in Korea. A cross-sectional study was performed to measure CA19-9 and CA125 among 76 healthy males and 95 healthy females in Korea. CA19-9 and CA125 were quantitated using solid-phase radioimmunoassay kits. Informations on the factors which might be related to the levels of these markers were collected by questionnaire(e.g., smoking, alcohol consumption, menstruation, oral pill use, breast- feeding history, etc.). There was no statistically significant difference in the mean of CA19-9 concentration between men(l0.4 u/ml) and women(l0.l u/ml), Whereas the mean of CA125 levels(11.2 u/ml) was higher in omen than that(2.5 u/ml) in men. Although there was a statistically significant association between CA19-9 and average number of cigarette consumed per day(r=0.59, p=0.026) and total number of cigarettes consumed in women(r=0.74, p=0.003), the significance disappeared by multiple regression analysis after adjusting age and body mass index. Later age of menopause(p=0.035) and longer duration f breast- feeding(p=0.050) were significant predictors for CA125 levels in women by multiple regression analysis after adjusting age and body mass index. In conclusion, CA19- 9 can be used as a stable tumor marker in clinical practices, however, menstruation and breast-feeding should be considered when CA125 is used in women.