The aim of this study was to detect mutant APC DNA of tumor origin in the plasma of patients with sporadic colorectal carcinomas. The polymerase chain reaction (PCR) and the single strand conformation polymorphism (SSCP) procedures were employed to detect DNA alterations using primers to amplify the mutation cluster region of the APC gene. APC mutations were observed in 7 out of 11 archival colonic tumor specimens examined. Matching mutations in free plasma DNA of tumor origin were detected in 3 of the 7 patients (42.8%). The results of this preliminary report indicated the presence of APC DNA in plasma harboring the identical abnormal molecular signature of tumor APC DNA. Detection methods of mutant APC DNA in blood may prove useful in the screening and monitoring of patients at risk of or with colorectal cancer.