Aromatase levels were measured in tumor tissue vis-a-vis clinico-morphological patterns of tumor, menopausal status and sex hormone concentration in blood in 50 patients with breast tumors. The test was based on heavy water release from 1 beta-3H-androstenedione. Direct correlation was established between aromatase concentration, on the one hand, and tumor size, cell differentiation status and blood-testosterone on the other. The data point to the role of aromatase concentration in situ during breast tumor genesis, its dependence on hormonal environment of tumor and its presence in breast tumor cells.