In order to elucidate the effect of supplying vessels on 5-FU distribution within a tumor, the correlation between 5-FU tissue concentration and tumor microangiography was examined in 31 patients with colorectal cancer after administration of 5-FU (500 mg/day for 3 days). The 5-FU concentration at the tumor margin was significantly higher than that in the other two tissue areas (p less than 0.05). A low 5-FU concentration was noted in the central region of ulcerative cancer showing vertical invasion with vascular disarray and/or histological necrotic foci. This type of tumor also showed a low RNA index by flow cytometry. The present study indicates that the vascular change and vascular pattern of a tumor may be a major contributing factor to 5-FU transfer from the blood circulation into a tumor.