Several studies have suggested gender differences in cognitive function, but data on the association between sex hormones and cognitive function are contradictory. The aim of our randomized double-blind study was to explore the possible relations between cognitive function and serum levels of sex hormones, oxytocin and insulin-like growth factor-I (IGF-I) in postmenopausal women. Two-hundred healthy postmenopausal women were randomly assigned to receive estrogen, testosterone or placebo treatment for 1 month. The associations of spatial ability, verbal fluency and verbal memory with serum levels of estradiol, testosterone, estradiol/testosterone ratio, androstanediol, oxytocin and IGF-I were analyzed. Spatial ability showed a negative correlation with serum estradiol, estradiol/testosterone ratio, oxytocin levels and a positive association with androstanediol levels. Verbal fluency displayed a negative relationship with serum levels of testosterone, IGF-I and a positive with estradiol/testosterone ratio. Verbal memory displayed a positive correlation to androstanediol. Data suggest that not only absolute levels of sex hormones but also the balance between estrogen and testosterone and their metabolites may be important for cognitive function in women.