Blood insulin level was measured in 113 breast cancer (BC) patients, 18 endometrial cancer (EC) patients, and 35 women with benign breast disease (BBD), after fasting and after 120 min of oral glucose tolerance test (OGTT). A significant increase in reactive insulin level was shown in postmenopausal BC patients with abdominal obesity (waist/hip ratio > 0.85) and no differences in insulin level were found between BC and BBD patients. Menstrual status and overweight (Quetelet index) did not influence significantly blood insulin concentration in BC patients, but the basal insulin level was lower in those patients who had been moderate smokers. In EC patients, the level of insulin after fasting and following 120 min OGTT was much higher than in BC and BBD patients although they had a similar body mass to these groups of patients. The effect of age on insulin secretion in BC patients is discussed as well as the possible causes and consequences of hyperinsulinemia developing in EC and BC patients.