Purpose To investigate the expression of DLC1 and its relationship with Ki-67 in cancerous and non-cancerous tissues of the breast.Methods In situ hybridization and immunohistochemiscal EnVision method were used to detect the expression of DLC1 mRNA and protein and Ki-67 in 52 invasive breast ductal carcinomas and 42 non-cancerous mammary tissues, including 22 mammary fibroadenomas and 20 paracancerous tissues.Results The positive rates of DLC1 mRNA and protein expression in the breast carcinomas (50% and 57.7%) was significantly lower than that in the non-cancerous mammary tissues (90.5% and 92.9%) (χ~2=17.518 and 10.729,P＜0.01).The expression of DLC1-mRNA was positively related to DLC1protein (r_s=0.379,P＜0.01). The positive rate of Ki-67 expression was 61.5% in the breast carcinomas, but no expression was observed in the all non-cancerous tissues (χ~2=39.186,P＜0.01).Correlation analysis showed that DLC1 expression was negatively correlated with Ki-67 expression (r_s=-0.507,P＜0.01).Conclusions Lower or no expression of DLC1 mRNA and protein may play an important role in the pathogenesis and progression in breast carcinoma. DLC1 may inhibit the proliferation of the breast carcinoma cells,which indicates that it may act as a new molecular marker of breast carcinoma.Combining detection of DLC1 and Ki-67 may be useful parameters for evaluating the biological behaviors of breast carcinoma.