Aim: We investigated the effect of the number of cytosine-adenine (CA) repeats in the ADAMTS9 promoter region on breast cancer lymphatic metastasis.Materials and methods: Thirty-one postoperative breast cancer patients were selected and examined retrospectively. The patients were classified into 2 groups: metastatic or nonmetastatic. Thirty healthy women were selected as the control group, and their peripheral blood was obtained. Following DNA isolation from the cancer tissue specimens and peripheral blood, the promoter region of the ADAMTS9 gene was directly sequenced and the number of CA repeats was determined.Results: The number of CA repeats ranged between 19 and 21 in the control and metastatic groups. However, in the nonmetastatic group, the number of CA repeats ranged between 17 and 18. This difference in the median number of CA repeats between the control group and the nonmetastatic group was statistically significant.Conclusion: A potential relationship may exist between lymphatic metastasis in breast cancer and the number of CA repeats in the promoter region of the ADAMTS9 gene. Our study indicates a potential association between the number of CA microsatellite repeats in the promoter region of the ADAMTS9 gene and breast cancer lymphatic metastasis.