We investigated the wide variability of 18F-2-fluoro-2-deoxy-D: -glucose (FDG) uptake, semiquantified as standardized uptake value (SUV), in positron emission tomography (PET) scanning, in 20 patients with colorectal cancer (CRC), including 1 with synchronous hepatic metastasis. The sensitivity of PET in CRC diagnosis was 100%, with a mean SUV of 8.0 (3.1-11.9). Tumor size and depth of invasion were associated with higher SUVs (P=.0004, .042, respectively). Strong glucose transporter-1 (GLUT-1) expression had significantly positive correlation with the SUV (r=.619, P=.003). GLUT-1 expression revealed positive staining in 17 (85%) of the 20 primary lesions. The central part of the tumor, thought to be relatively hypoxic, had stronger GLUT-1 expression and a higher SUV than the periphery, in both the primary tumor and hepatic metastatic foci. Our data suggest that the SUVs of FDG uptake in PET may be a noninvasive biomarker for advanced CRC, indicative of a large hypoxic tumor with deep invasion.