Peripheral aromatase activity was measured in 24 postmenopausal women suffering from advanced breast cancer. The % conversion of androstenedione to oestrone was then assessed for a significant correlation with age, weight, height, Quetelets index (weight/height2, Q.I.) and length of menopause. Serum oestradiol (E2) levels were measured in 22 of the subjects and compared with the same indices. There was no correlation between E2 or aromatase activity with the length of menopause (P = 0.3 and P = 0.5, respectively). In our data aromatase activity did not correlate with age (P > 0.5, n = 22). Serum E2 levels (P = 0.07, n = 20) expressed a negative correlation (i.e. decreased) with age. There was also a poor correlation between aromatase activity and weight of Quetelets index (P = 0.3, n = 20 for both). Serum E2 levels showed a statistically significant correlation with weight (P = 0.01, n = 21), but the relationship with Quetelets index just failed to attain statistical significance (P = 0.07, n = 20). In both cases the regression line was positive. When aromatase activity was correlated with serum E2 levels the regression line was positive but not statistically significant (P = 0.4, n = 22). The data indicate that aromatase activity is only one factor determining the differences in serum E2 levels between postmenopausal women.