Selective internal radiation therapy (SIRT) with radiolabeled Yttrium-90 (90Y) microspheres is a relatively new therapeutic option for patients with primary or secondary hepatic malignancy. However, the evaluation of the therapeutic efficacy on anatomic imaging alone is suboptimal. Functional imaging with F18-2-fluoro-2-deoxy-D-glucose (FDG) positron emission tomography (PET) is increasingly utilised in therapy monitoring in malignancy and provides an accurate assessment of metabolic response. We report a case of a 75-year-old gentleman with colorectal hepatic metastases who demonstrated a dramatic metabolic response on PET scan following SIRT. This case illustrates the utility of FDG PET in the assessment of the therapeutic efficacy of 90Y microspheres in colorectal hepatic metastases.