The hormone profiles of nulliparous and parous women on day 11 of their menstrual cycle have been studied in an attempt to seek evidence for the hormonal basis of the protective effect of first birth on breast cancer risk. A previous publication reported that there were significantly lower (26%) early morning prolactin levels in parous women as compared to those levels in nulliparous women but that there were no differences in plasma and urinary estrogen levels. The present study shows, however, that parous women had significantly shorter cycle lengths than nulliparous women of the same age, and the data were reevaluated with this difference being taken into account. After adjustment for cycle length (within the range of 24-32 days) and age, estrogen levels were significantly lower (22%) in parous women compared to those in nulliparous women. Two further aspects of estrogen metabolism were measured in the plasma samples of these women: the binding capacity of sex hormone-binding globulin (SHBG) and the percentage of free estradiol (E2). SHBG levels were 12% higher in parous women, but there was no difference in percentage of free E2. In the previous publication it was suggested that the protective effect of first birth on breast cancer risk was mediated in part by permanently lowering prolactin levels. The current findings suggest that changes in estrogen metabolism also are a factor.