Stromal cells and adipocytes were separated after collagenase treatment of adipose tissue obtained from women undergoing elective surgery, and these cells were used to study aromatization of [3H]androstenedione in vitro. Aromatization activity was estimated either 1) by determining the incorporation of tritium from [1-3H]androstenedione into [3H]water or else 2) by determining the formation of [3H]estrone (E1) and [3H]estradiol (E2) from [1,2,6,7-3H]androstenedione. It was established that only 13% of the aromatase activity of adipose tissue resided in the adipocyte fraction, whereas 87% of the aromatase activity was in the stromal/vascular fraction. Subsequent studies of aromatization were conducted utilizing stromal/vascular cells grown to confluence in monolayer culture. In such cells, the formation of [3H]E2 was slower initially but increased with time, and after 48 h of incubation, the amount of [3H]E2 produced exceeded that of [3H]E1. The rate of [3H]E1 formation, as a function of [3H]androstenedione concentration, followed Michaelis-Menten kinetics. The Vmax ranged from 0.8-3.0 pmol and ranged from 0.16-0.67 pmol mg-1 cell protein 6 h-1 in cells from omental adipose tissue. The apparent Km for [3H]androstenedione in stromal cells derived from both omental and sc tissue was the same, i.e. about 25 nM. We conclude that the ability of human adipose tissue to form estrogen is not a function primarily of the adipocytes but rather resides principally in the cells of the stroma.