We investigated S-adenosylhomocysteine hydrolase-like protein 1 (AHCYL1) gene expression in human epithelial ovarian cancer (EOC) using the chicken, which is the most relevant animal model. Ovarian cancer was detected in 10 of 136 laying hens (7.4%). Results of the present study indicated that AHCYL1 mRNA and protein are most abundant in the glandular epithelium of adenocarcinoma of cancerous, but not normal, ovaries of hens. In addition, bisulfite sequencing to examine methylation patterns in the promoter region of the AHCYL1 gene revealed that 30-38% of the three CpG sites were demethylated in ovarian cancer cells as compared with normal ovarian cells. Furthermore, in human ovarian cancer cells such as OVCAR-3, AHCYL1 protein was predominantly in the nucleus and had a similar expression pattern to that in chicken ovarian cancer cells. Thereafter, we examined the prognostic value of AHCYL1 expression in patients with EOC using multivariate linear logistic regression and Cox's proportional hazard analyses. In 109 human patients with EOC, 14 (12.8%), 41 (37.6%) and 54 (49.6%) patients showed weak, moderate and strong expression of AHCYL1 protein, respectively. However, intermediate or high expression of AHCYL1 protein was a favorable factor for overall responses (adjusted odds ratio, 7.23; 95% confidence interval [CI], 1.36-38.39), and for progression-free survival (adjusted hazard ratio, 0.20; 95% CI, 0.07-0.55). From these results, we conclude that AHCYL1 expression is associated with ovarian carcinogenesis as an oncogene in chickens, whereas it plays the role of tumor suppressor in human EOC, suggesting a paradoxical function of AHCYL1 in ovarian carcinogenesis.