Epidemiological data have suggested a possible relationship between obesity, diabetes mellitus and cancer risk, particularly breast cancer. We set out to investigate the effect of body mass index and diabetes mellitus on the presence of breast cancer in the Apulian population. We selected 1,663 women affected with primary breast cancer and 4,702 control patients. All patients with breast cancer underwent surgical excision of the tumor and their tumors were histologically confirmed. The prevalence of type 2 diabetes (8%) in the women affected by breast cancer was significantly higher than in the control group (5%) (p < 0.05). The majority of the diabetic women affected by breast cancer had a BMI value > 25, both in premenopause and in postmenopause. With respect to BMI, the non-diabetic patients with breast cancer in postmenopause showed the same pattern as the diabetic ones. Instead, among the women in premenopause a higher percentage (55%) of patients with a BMI < 24.9 was found (p < 0.01). In the Apulian population, the presence of both type 2 diabetes and elevated values of BMI (that is in a condition of hyperinsulinemia) were found to enhance the frequency of breast cancer.