To explore the relationship between extraglandular estrogen formation and the clinical parameters of age, weight, and endometrial neoplasia, an assay was developed to measure the rate of estrogen formation from androgen precursors in human adipose tissue. The substrate, [3H]androstenedione, was more efficient than [3H]testosterone for the aromatase system in this tissue. The highest rate of conversion of androgen to estrogen occurred in tissue slices rather than in minces or microsomal preparations. The addition of cofactor NADPH was not required for aromatization in slices or minces, probably as a consequence of endogenous production of NADPH by the tissue. [3H]Estrone was the sole estrogenic product identified in incubations when [3H]androstenedione was used as the substrate, whereas [3H]estradiol-17/β was the exclusive estrogen detected when using [3H]testosterone. The rate of [3H]estrone formation from [3H]androstenedione in tissue slices was linear with time of incubation up to 2 h and with tissue ma...