C19 steroids are converted to estrogens in a number of tissues by a specific form of cytochrome P450, namely aromatase cytochrome P450 (P450arom). Adipose tissue is the principal site of estrogen formation in postmenopausal women. Aromatase activity as well as P450arom transcripts primarily reside in the stromal cell component of the adipose tissue. Studies designed to investigate whether increased local aromatase activity in breast adipose tissue influences the growth of breast cancers have yielded discrepant results. In an attempt to clarify this controversy, adipose tissue was obtained from the four breast quadrants at the time of mastectomy (n = 13) performed for removal of a tumor. Breast fat P450arom messenger RNA levels were quantified and compared between the four quadrants within each specimen using competitive polymerase chain reaction after reverse transcription in which 10 micrograms human adipose total RNA together with 1 pg rat complementary RNA (internal standard) were reverse transcribed and coamplified. In 9 out of 13 patients (69%), highest P450arom transcript levels colocalized to the quadrants bearing tumors. This correlation was statistically significant (P < 0.001). The regional distribution of P450arom transcripts in breast adipose tissue of disease-free individuals, obtained during reduction mammoplasty (control group, n = 9), did not favor any particular region of the breast. We also quantified by morphometry the histological components of the adipose tissue samples from each quadrant in mastectomy specimens. The distribution of stromal cells significantly correlated with the distribution of P450arom transcript levels, in that quadrants containing highest proportions of stromal cells matched to highest transcript levels (P < 0.01). Although the quadrants bearing tumors contained the highest percentage of stromal cells, this correlation was not statistically significant. The adipose tissue surrounding a breast tumor displays increased estrogen biosynthesis, which may promote tumor growth. It is further suggested that the distribution of stromal cell components in breast adipose tissue gives rise to locally elevated P450arom expression, which in turn may favor neoplastic development and growth in these predisposed areas of the breast. The correlation between the presence of a tumor and elevated P450arom levels in the proximal adipose tissue is independent of tumor size, node involvement, histological type or grade, estrogen/progesterone receptor status, DNA index, or S-phase fraction.