OBJECTIVE The tumor stage represents the single most important prognostic factor for colorectal cancer (CRC), although more accurate prognostics remain much needed. Previously, we identified CA125 as an independent significant prognostic factor, which we have further validated along with CEA, CA19-9, and CA242 in a large cohort of CRC patients. METHODS Using enzyme-linked immunosorbent assays, we analyzed preoperative serum samples in 322 CRC patients operated on between 1998 and 2003. RESULTS Using the Spearman's rho model, we calculated the correlation between our previous findings on MUC16 and CA125, for which the correlation coefficient was 0.808 (p < 0.001). The Cox regression analysis of the linear and logarithmic values of CEA, CA125, CA242, and CA19-9 identified only CA125 (hazard ratio [HR] 1.03; 95% confidence interval [95% CI] 1.02-1.04; p < 0.001) as significant when using the linear values. Survival among CRC patients with a high CA125 level was poor compared with CRC patients with a low CA125 level (HR 2.48; 95% CI 1.68-3.65; p < 0.001). In subgroup analyses, patients with high CA125 levels and aged ≤67 or >67, with stage I-II or III-IV, and both colon and rectal cancer exhibited poor prognoses. In the multivariate analysis, we used clinical pathological variables in the model, where age, gender, and stage served as the background characteristics. We dichotomized CA125 using the Youden maximal cutoff point, and the median values for CEA, CA19-9, and CA242. CA125 emerged as the only marker remaining significant and independent together with stage, location, and age (HR 1.91; 95% CI 1.24-2.95; p 0.003). CONCLUSIONS CA125 represents a significant and independent prognostic factor in CRC patients, superior to CEA. Furthermore, CA242 served as a better prognostic marker than both CEA and CA19-9. We recommend including both CA125 and CA242 in prognostic clinical trials among CRC patients.