The ability of human abdominal, breast and axillary fat to convert androgens into estrogens was investigated by incubating labeled substrates in the presence of NADPH with a variety of cell preparations. The incubation products were subjected to phenolic partition, paper chromatography, methyl-ether formation, repeat chromatography and crystallization with cold carrier reference standards to constant specific activity. Androstenedione was converted to estrone and, to a lesser extent, to 17β-estradiol by crude homogenates, minces, fat-free particulate fractions (1,000–100,000 × g) and isolated fat cells obtained from abdominal, breast or axillary fat. Testosterone was found to be aromatized as actively as androstenedione, but in this case more 17 β-estradiol was formed than estrone. 19-Hydroxyandrostenedione² also served as substrate, giving results similar to those obtained with androstenedione. Fat tissue obtained from cancerous breasts was found to be as active as normal breast fat (1–4 pg/g fat/90 min) and within the range found for abdominal fat (1–27 pg/g fat/90 min). In each case in which axillary fat was compared to breast fat from the same subject, the activity of the axillary fat was 5 to 10 times higher. The results indicate a possible role of adipose tissue as a significant extra-gonadal source of estrogens.