Detection of tumor DNA in peripheral blood of patients with colorectal cancer (CRC) may allow early diagnosis of tumor disease and be of prognostic value. However, a reliable tumor marker detectable in the serum of patients with this disease is missing. Because k-ras and APC mutations occur frequently and at an early stage in CRCs, these mutations might also be detected in the serum of CRC patients and serve as tumor markers. Hence, tumor tissues of CRC patients were examined for the presence of mutations in the k-ras and APC genes. If a mutation was detected in the tumor, the serum of the patient was screened subsequently for the presence of this mutation. K-ray mutations were detected in 22 of 30 colorectal tumor tissues, but only in six patients was the mutation identified in their serum samples. Mutations of the APC gene were identified in 25 of 65 tumors: 20 of these 25 patients showed the respective mutation in their serum. Given their higher detection rate, APC mutations could be a more informative serum marker than k-ras in CRC patients.