PURPOSE: To evaluate the relative value of arterial, portal and delayed phase images in the measurement of hepatic metastatic mass arising from gastrointestinal malignant tumor using spiral CT. MATERIALS AND METHODS: Thirty-three with 45 metastatic tumors of the liver underwent tri-phasic spiral CT. For this purpose one or two lesions were chosen in each patient whose primary tumor was shown to be stomach cancer(n=15), colon cancer(n=16), or ileal cancer(n=1). Tumor size ranged from 1 to 12.2 (mean, 4.3)cm. Arterial, portal and delayed phase images were obtained at 30 -35 seconds, 70 -75 seconds, and 3 minutes, respectively, after the injection of contrast materials. Using a work station, two radiologists independently measured the longest diameter of the selected lesions, and a second measurement was taken three days later. Contrast, as well as intra-and interbserver differences among the three phases, was statistically analysed. RESULTS: Intra- and interobserver difference were, espectively, 2.3 and 3.8 mm during the portal phase; 3.3 and 4.6 mm during the arterial phase; and 2.9 and 4.5 mm during the delayed phase. ANOVA with Tukey's multiple comparison showed that none of these differences were statistically significant. Contrast between mass and liver parenchyma was especially clear during the portal phase (p=0.0001, using the Kruskal-Wallis CONCLUSION: Intra- and interobserver differences in the measurement of hepatic metastatic tumors were statistically insignificant during all three phases. The least difference and best contrast were seen during the portal phase.