The preoperative serum levels of carbohydrate antigen 19-9 (CA 19-9) were determined in 206 patients with colorectal cancer, 52 (25.2%) of whom were found to be positive. All of these patients had advanced cancers and significantly higher incidences of tumor invasion through the muscularis propria (91.3%) and lymph node involvement (54.5%). The incidences of liver metastasis and Dukes' stage D in the CA 19-9-positive group were 38.5% and 42.9%, respectively, significantly higher than those in the CA 19-9-negative group of 6.5% and 14.8%, respectively. Moreover, the incidence of liver metastasis in the CA 19-9-positive group patients with Dukes' stage D cancer was 95.2% (20/21); CA 19-9 showing higher specificity (81.7%) and a more positive predictive value (38.5%) for liver metastasis than the carcinoembryonic antigen (CEA). When a cutoff value of 160 U/ml was used, the specificity and positive predictive value reached 97.7% and 81.0%, respectively. An analysis of response operating characteristic (ROC) curves for liver metastasis revealed that CA 19-9 was more useful than CEA. The long-term survival of the CA 19-9-positive group patients was significantly worse than that of the CA 19-9-negative group patients (P < 0.0001), with no 1.25-year survivors in the former group when the cutoff value of 160 U/ml was used. These results suggest that serum CA 19-9 as a useful preoperative indicator of liver metastasis and prognosis in colorectal cancer.