Obesity is associated with hyperleptinemia and this has led to the suggestion that leptin maybe a factor in cancer progression. To study the effect of leptin on cancer progression we used a mouse model of diabetes that was shown to enhance tumor progression and thereby determine if leptin affects cancer progression despite improvements in metabolic status. Mammary tumors were allowed to develop in male and female mice following orthotopic injection of cells expressing oncogenes. After 2 weeks leptin was administered to the mice using Alzet pumps. In these mice leptin failed to stimulate tumor progression; indeed, in those studies where glucose tolerance improved tumor growth was actually inhibited. Thus, the possibility exists that the effect of leptin on tumor progression maybe opposed by improvements in metabolism.