Autotaxin (ATX), originally isolated from human melanoma cells, is a novel metastasis-enhancing motogen and angiogenesis factor. In the present study, we compared the expression level of ATX mRNA between normal and breast cancer tissues and found that the expression of ATX mRNA was closely linked to invasiveness of cancer cells. Reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemical analysis showed higher cellular ATX mRNA expression in the cancer than normal breast tissues. MDA-MB-435S breast cancer cells, expressing higher amount of ATX mRNA, showed greater relative invasiveness to fibroblast-conditioned medium (FCM) than MCF7, MDA-MB-231, and HBL-100 breast cancer cells. Furthermore, ATX-transfected MCF7 cells showed increased motility and invasiveness than vector-transfected MCF7 cells. Collectively, our results suggest that the expression of ATX is closely linked to the invasiveness of breast cancer cells.