By separating 5'-nucleotide phosphodiesterase isoenzyme-V (5'-NPD-V) as a fast-moving isoenzyme by polyacrylamide electrophoresis, the determination of serum 5'-NPD-V was performed in 302 preoperative patients with gastric and colorectal cancers to assess the clinical usefulness for suspecting liver metastases. Serum levels of CEA, alpha-fetoprotein and tumor markers were simultaneously measured. Angiography, CT scan and echo were also performed preoperatively. The normal values of serum 5'-NPD-V in 67 healthy subjects except heavy smokers were less than 3.0mm. 5'NPD-V values determined in patients with and without liver metastases were as follows: In gastric cancer 1.5 +/- 2.0mm and 8.6 +/- 9.0mm, and in colorectal cancer 2.2 +/- 3.3mm and 5.8 +/- 5.3mm, respectively, indicating a significant difference (p less than 0.05). The sensitivity of 5'-NPD-V in gastric cancer was 0.682, the specificity was 0.892, the predictability was 0.518, and the accuracy was 0.862. The results in colorectal cancer were 0.600, 0.958, 0.805 and 0.800, respectively. Serum 5'-NPD-V value was elevated progressively in accordance with extent of liver involvement. When assessed by 5'-NPD-V and CEA, 80.9% of patients with liver metastases proved to be correctly diagnosed. The results suggest that 5'-NPD-V is clinically a useful marker in that the isoenzyme provides the rationale for the further detection of tumor localization in the liver.