Increased physical activity and programs to reduce body mass index (BMI) with both increased physical activity and decreased caloric intake have been proposed to reduce insulin as a potential mediator of breast cancer and other chronic diseases. However, there are few data on the relative contribution of physical activity, caloric intake, and BMI to fasting insulin levels. An ethnically diverse subsample of 2,996 mostly healthy postmenopausal women with no prior cancer history was randomly identified from the 161,809 participants in the Women's Health Initiative clinical trials and observational study. Information was collected on diet, recreational physical activity, and anthropometrics including BMI. Fasting insulin levels were determined. Using a cross-sectional design, insulin levels were then compared across quintiles of caloric intake and physical activity in linear regression model analyses controlled for BMI and other factors. Lower BMI (P < .0001), higher levels of physical activity (P < .0001), and lower caloric intake (P < .02) were all independently associated with significantly lower mean fasting insulin levels throughout the range of observed values. Insulin levels of 8.74 microU/mL +/- 4.16 SD were seen in the highest physical activity and lowest caloric intake quintile compared with insulin levels of 15.08 microU/mL +/- 16.32 SD in the lowest physical activity and highest caloric intake quintile (P < .0001). These findings suggest that reduction in BMI achieved by increasing physical activity, reducing caloric intake, or both, should lower insulin levels, providing support for clinical trials evaluating insulin level change and breast cancer risk.