Given that existing epidemiologic data on the correlation between dietary fat and breast cancer have been mixed, the Women's Intervention Nutrition Study was launched in 1987. This randomized clinical trial of 2437 women between the ages of 48 and 79 y with early-stage breast cancer tested the hypothesis that dietary fat reduction would increase the relapse-free survival rate. The study determined that low-fat dietary interventions can influence body weight and decrease breast cancer recurrence. Results showing a differential effect of diet on hormone-receptor-positive and -negative disease suggest that metabolic mechanisms involving insulin and insulin-like growth factor-1 may be involved in tumorigenesis. The results of the Women's Intervention Nutrition Study may therefore contribute to knowledge of the role of insulin resistance in cancer risk.