PURPOSE: Free radicals are defined as any species capable of independent existence that contains one or more unpaired electrons and they have the effects on carcinogenesis and tumor progression by causing mutations on genetic structure or suppressing repair of mutated DNA. This study was aimed to identify changes of antioxidant capacity in carcinogenesis and tumor progression of colorectal cancer (CRC). METHODS: We studied the level of serum antioxidant capacity using ABTS technique for 34 CRC patients who were operated between July 1997 and January 1998 at department of surgery, Taegu Catholic Medical Center and compared with 38 persons who had normal value of liver function during the same period. RESULTS: CRC patients showed decreased serum antioxidant capacity level compared to that of control group (CRC male 1.23 0.15 mmol/L, CRC female 1.11 0.13 mmol/L, control male 1.40 0.15, control female 1.35 0.11 mmol/L). Changes of antioxidant capacity levels were not correlated with stages, even though the marginal difference between T-stages (T1/2 1.23 0.10 mmol/L, T3/4 1.16 0.15 mmol/L). CONCLUSIONS: Free radicals may be the causative agent of colorectal carcinogenesis and can be associated with early phase of carcinogenesis rather than tumor progression.