BACKGROUND The published results on expression of c-kit in benign and malignant breast tissues vary. MATERIALS AND METHODS The immunohistochemical expression of c-kit proto-oncogene product in 52 invasive breast cancer tissues and 16 benign breast tumor (fibroadenoma) tissues was studied using anti-c-kit proto-oncogene product antibody. Its expression was evaluated by immunoreactive score (IRS). RESULTS In breast cancer tissues, the mean IRS of c-kit proto-oncogene product expression was significantly increased compared to those of fibroadenoma (3.4 +/- 2 and 2.19 +/- 1.8, respectively,p = 0.035). The mean IRS of c-kit expression was higher in the group comprising estrogen (ER) positive tumor than in the group of ER negative (4.1 +/- 2.1 and 2.7 +/- 1.8, respectively,p = 0.012) but no statistically significant relationship was seen between the expression of c-kit proto-oncogene product and other clinicopathological parameters of breast cancer, including histologic type, tumor size, lymph node metastasis, distant metastasis, stage, progesterone receptor, c-erbB-2 expression, menopausal status and age of the patient (p > 0.05). CONCLUSIONS Our results show that a high level of c-kit expression occurs frequently in invasive breast cancer, and its expression is associated with ER but unrelated to other clinico-pathological variables.