Objective: This study sought to test whether a biological measure of chronic stress, Δ cortisol, was related to BMI and whether the relationship between Δ cortisol and BMI varied according to education and positive affect. Research Methods and Procedures: One hundred fifty-four women from a blue-collar women's health project in 11 industrial sites in rural North Carolina provided saliva for cortisol assays for a substudy on "stress." Δ Cortisol, the difference between awakening and midday cortisol measures representing diurnal decline, was calculated (lower values = greater stress). BMI was regressed on A cortisol, education, and positive affect. Analyses were controlled for age, race, and worksite. Standardized β-coefficients were calculated. Results: For participants with complete data (n = 129), BMI was greater (β; 95% confidence interval) for women with less than high school education (0.56; 0.18, 0.94) and those who completed high school (0.26; -0.05, 0.57) relative to women with greater than a high school education (p = 0.009). Δ Cortisol was inversely related to BMI (-0.32; -0.59, -0.05; p = 0.022). Education positively modified the inverse relationship between A cortisol and BMI (p = 0.047). Positive affect was negatively associated with BMI (-0.44; -0.82, -0.06; p = 0.026) and positively modified the inverse association between A cortisol and BMI (0.33; -0.03, 0.69; p = 0.074). Discussion: Education and Δ cortisol were inversely related to BMI, and the magnitude of the association between Δ cortisol and BMI was buffered by higher education. Positive affect was inversely related to BMI. Chronic stress is associated with higher BMI, with this relation attenuated by higher education and, possibly, a positive affect.