Leptin is an adipocyte-derived protein and plays an important role in the control of body weight by acting as a neurohormone regulating energy balance and food intake in the hypothalamus. The high serum leptin levels and the overexpression of leptin receptors have been documented in breast cancer patients, but the levels never checked preoperatively. In the present study, the relationship between preoperative serum leptin levels of the breast cancer patients and the healthy controls were evaluated. The serum leptin levels in 30 breast cancer patients were compared to 30 healthy female volunteers. In addition, the association of serum leptin levels and the various well-known risk factors were studied. Serum leptin levels of patients with breast cancer (28.55 + 19.7 ng/ml) were tended to be higher than those of controls (26.43 + 19.4 ng/ml), but it did not reach statistical difference (P = 0.712). There was significant correlation between the expression of ER, PR, and serum leptin levels (P = 0.018 and 0.037, respectively), but not with the HER-2/neu receptor expression (P = 0.067). Also association was not found between the tumor size, lymph node involvement, and the levels of serum leptin (P = 0.235, 0.34, and 0.86, respectively). The serum leptin level was also found to be similar in premenopausal (24.85 +/- A 18.14 ng/ml) and postmenopausal (30.49 +/- A 17.19 ng/ml) patients (P = 0.235). The preoperative serum leptin levels in breast cancer patients were similar to healthy controls. In subset analysis, the significant correlation between the leptin level and hormonal status was noted, but association with HER-2/neu was not detected. These findings should be confirmed with larger studies.