We have examined the serum levels of the mutant p53 protein in patients with colorectal cancer preoperatively (n=50), and in patients with adenomatous polyp (n=13). Mutant p53 protein in patients after curative surgical resection of colorectal cancer (n=26, part of the fifty preoperative patients) was also measured. Serum samples were stored frozen at -70℃ until the time of analysis. We used the p53 mutant ELISA (QIA03, CALBIOCHEM) system. Serum levels of the mutant p53 protein in patients with colorectal cancer (mean=0.97±0.14 ng/ml, ranged from 0.7 ng/ml to 1.37 ng/ml, n=50) were significantly greater than those in patients with adenomatous polyp (mean=0.73±0.06 ng/ml, ranged from 0.69 ng/ml to 0.83 ng/ml) (p<0.001). There was a significant correlation between serum p53 levels and CA19-9 levels (p<0.01). Serum levels of the mutant p53 protein prior to surgery (mean=0.97±0.13 ng/ml, n=26) significantly decreased after surgical resection of tumor (mean=0.82±0.07 ng/ml) (p<0.001, paired t-test). These results suggest that mutant p53 protein might be used as a potential biomarker in the management of patients with colorectal cancer. Further study is warranted to establish its clinical significance.