Endogenous estrogens play an important role in women's organism: they control menstrual cycle through the influence on foliculogenesis, ovary's steroidogenesis and growth, and endometrium transformations. But their role is not only confined to ovaries and testis action. Number of researches report about systemic role of these hormones in women. They are involved in lipids and carbohydrates metabolism, bone mineralization, vascular functions. Estrogens are synthesized not only in gonads, but also in adipose cells, bones, brain, vasculature and adrenal cortex. Many enzymes are involved in estrogen biosynthesis pathway. It seems that two of them are one of the most important aromatase cytochrome P450, complex of the 17a-hydroxylase and 17,20-lyase. They are products of the CYP19 and CYP17 genes respectively. The CYP19 gene encodes the aromatase cytochrome P450 enzyme, which is responsible for final steps in biosynthesis of estrogens. The CYP17 gene encodes complex of the 17a-hydroxylase and 17,20-lyase enzymes, which catalyze transformation from pregnenolone and progesterone to DHEA and androstendione respectively, which are major precursors of the estrogens. A number of publications report about influence of the genetic variation across these genes on reproductive system functions such as estrogen concentration, age at the natural menopause, breast and endometrial cancer risk in women.