Human arrest-defective-1 (hARD1) was reported to be important in regulating cell cycle and promoting lung cancer cell proliferation. Here we have investigated the correlation between hARD1 and breast cancer. Analysis with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and flow cytometry (FCM) demonstrated that overexpression of hARD1 was associated with increased proliferation of MCF-7 cell, a human breast cancer cell line. Western blotting and immunohistochemical staining assay showed that hARD1 presented higher in breast cancer tissue than the adjacent tissue; accumulation of hARD1 protein was higher in 86% (37/43) of breast cancer, far more than noncancer samples. Our results suggest that hARD1 might play an important role in breast cancer carcinogenesis.