Eleven flavonoid compounds were compared with aminoglutethimide (AG), a pharmaceutical aromatase inhibitor, for their abilities to inhibit aromatase enzyme activity in a human preadipocyte cell culture system. Flavonoids exerting no effect on aromatase activity were catechin, daidzein, equol, genistein, β-naphthoflavone (BNF), quercetin and rutin. The synthetic flavonoid, α-naphthoflavone (ANF), was the most potent aromatase inhibitor, with an I 50 value of 0.5 μM. Three naturally-occurring flavonoids, chrysin, flavone, and genistein 4′-methyl ether (Biochanin A) showed I 50 values of 4.6, 68, and 113 μM, respectively, while AG showed an I 50 value of 7.4 μM. Kinetic analyses showed that both AG and the flavonoids acted as competitive inhibitors of aromatase. The K i values, indicating the effectiveness of inhibition, were 0.2, 2.4, 2.4, 22, and 49 μM for ANF, AG, chrysin, flavone, and Biochanin A, respectively. Chrysin, the most potent of the naturally-occurring flavonoids, was similar in potency and effectiveness to AG, a pharmaceutical aromatase inhibitor used clinically in cases of estrogen-dependent carcinoma. These data suggest that flavonoid inhibition of peripheral aromatase activity may contribute to the observed cancer-preventive hormonal effects of plant-based diets.