Alterations in several classes of adhesion molecules have been implicated in the progression of colorectal cancer. Cell adhesion regulator (CAR) has been identified as a regulator molecule of integrin-dependent cell adhesion. We have explored a possible involvement of the CAR gene in colorectal cancer. Reverse transcription-PCR revealed that CAR expression was detected in normal colonic cells, whereas it was decreased or undetectable in 6 of 13 (46.2%) human colon cancer cell lines. To further study the biological significance of CAR expression in colon cancer cells, a CAR expression vector was introduced into HT-29 cells, in which CAR is not expressed. Adhesion of HT-29 cells to extracellular matrix components was up-regulated by the introduction of CAR. In spite of similar growth properties with the controls, CAR-transfected HT-29 cells showed a significantly reduced spontaneous metastatic potential in nude mice. To determine whether these experimental results are of relevance with respect to actual human tumors, we investigated CAR expression in 30 surgical specimen pairs of human colorectal cancer and adjacent noncancerous tissue using semiquantitative reverse transcription-PCR. In 14 of 30 cases (46.7%), CAR expression in cancer was less than one-tenth of that in matched noncancerous tissue. The tumor:normal ratio of CAR expression was significantly lower in patients with lymph node metastasis than in those without it (P < 0.01) and in patients with distant metastasis than in those without it (P < 0.05). CAR expression was significantly lower in more advanced Dukes' stage tumors (P < 0.05). Our results suggest that down-regulation of CAR expression may play an important role in the progression and metastasis of colorectal cancer.