Arachidonic acid metabolism plays an important role in colon carcinogenesis. Cyclooxygenase-2 (COX-2), which catalyzes the rate-limiting step in the synthesis of prostaglandins from arachidonic acids, is known to be up-regulated in colon cancer, and multiple lines of evidence indicate that it is a critical early step in colon carcinogenesis. Recently, 15-lipoxygenase-1, the enzyme that converts arachidonic acid to 15(S)-HETE, was also found to be up-regulated in colon carcinoma. In our previous studies, we cloned a gene that encodes another arachidonic acid-using enzyme, fatty acid CoA ligase 4 (FACL4), and showed that overexpression of this enzyme prevents apoptosis. We have also showed that FACL4 and COX-2 synergistically inhibit apoptosis by reducing the intracellular level of free arachidonic acid. Here, we report that expression of FACL4 is significantly increased in colon adenocarcinoma compared with adjacent normal tissue at both the mRNA and protein levels by quantitative RT-PCR (paired t test, P < 0.015), immunoblot, and immunohistochemical staining. We found that the increase in expression level of FACL4 mRNA relative to control ranged between 2.4- and 54.5-fold; the average fold-increase was 13.4. The increase in FACL4 protein expression is between 2.4- and 65.0-fold. In addition, we found that a higher level of increased FACL4 expression was correlated with well and moderately differentiated adenocarcinoma, whereas no similar correlation was observed with COX-2 expression. The in situ hybridization results indicate that expression of FACL4 is localized predominantly in the colon epithelium but not in the stroma. The onset of FACL4 up-regulation appears to occur during the transformation from adenoma to adenocarcinoma because FACL4 expression was not increased above normal in the three colon adenomas examined. Finally, we observed that a tumor promoter significantly induced FACL4 expression. These findings suggest that the FACL4 pathway may be important in colon carcinogenesis, and that the development of selective inhibitors for FACL4 may be a worthy effort in the prevention and treatment of colon cancer.