Purpose: To examine the relationship between plasma levels of reproductive sex steroid hormones in postmenopausal women and their reported fat intake. Methods: We measured plasma sex steroid hormanes levels in plasma collected in 1989 and 1990 from 381 healthy postmenopausal women. For each woman, we measured fat intake in 1986 and 1990 by a food-frequency questionnaire. The cross-sectional associations between the percentage of energy from total and specific types of dietary fat intake and plasma hormone levels were assessed by linear regression, controlling for energy intake, obesity, and protein intake. Results: The plasma estradiol level was 4.3% lower (95% confidence limits, -8.3%, -0.2%) for a substitution of 5% of energy from fat intake for an equivalent amount of energy from carbohydrate when adjusted for obesity and other covariates. Estradiol was also inversely associated with all other fat types except trans fat; the inverse associations with vegetable fat and marine omega-3 fats were statistically significant. Conclusion: We observed an inverse association between total fat intake averaged over 4 to 5 years and estradiol levels. This result is inconsistent with the hypothesis that fat intake predisposes to breast cancer risk by raising endogenous estrogen levels. (C) 2000 by American Society of Clinical Ontology.