Little to no data exists in the literature for serum estriol values in non-pregnant, premenopausal women. The current medical community opinion holds that estriol has no significant role in non-pregnant women relative to the other estrogens. It is a possibility that estriol's primary function has yet to be discovered. Accordingly, the first step is to understand cycle-dependent serum estriol concentrations. We have made a preliminary investigation for serum estriol concentration of 26 women during the known cycle peaks of estrone and estradiol. Five of the women were also tested for serum estriol on various days throughout the cycle in order to develop a cycle-dependent concentration profile. The result of these experiments show that serum estriol was always significantly higher than the sum of estrone and estradiol and less fluctuating. We conclude that estriol is probably a significant estrogen component.