Introduction: Positron emission tomography (PET) detects occult metastatic disease in up to 20% of patients with isolated hepatic colorectal metastases. However, the majority of patients do not benefit from PET scanning. We sought to evaluate the utility of PET scanning in patients with isolated colorectal hepatic metastases and correlate results with a clinical risk score (CRS). Methods: Patients with isolated colorectal hepatic metastases on conventional axial imaging who were imaged with PET scan between 1998-2002 were identified from a prospective PET database. A retrospective analysis of clinical parameters, radiological imaging results, intraoperative findings, and outcome was performed. All patients were assigned a clinical risk score, with one point added for each of five preoperative factors (disease-free interval <1 year, tumor size >5 cm, tumor number >1, CEA >200, node positive primary). Results: PET scanning was utilized in the preoperative evaluation of 85 patients. Radiological evaluation included contrast-enhanced abdomen/pelvic CT in 98%, chest CT in 38%, and abdominal MRI in 13%. In the majority of patients (53%) PET provided no additional information over conventional imaging. Overall, PET resulted in either detection of occult extrahepatic disease or confirmation of questionable findings on conventional imaging in 20% of patients, whereas PET readings were inaccurate in 27% (median F/U, 12 months). In order to determine if patient selection for PET could be enhanced to increase the yield, we evaluated the correlation of PET scan readings with CRS. There was a significant association of CRS with the yield on PET scan, such that in patients with a CRS of 0, 50% had false positive readings and no patient had extrahepatic disease detected with PET, while in patients with a CRS of ³ 1, there were no false positives and 10/77 (13%) were found to have extrahepatic disease detected only with PET scanning (Table, p<0.001, Fisher exact test). Conclusion: The clinical risk score correlated with the yield of PET, with a greater proportion of patients at low risk (CRS = 0) having false positive readings on PET scan. In addition, no patient with a CRS of 0 was found to have occult extrahepatic disease on PET scan. Patients with isolated hepatic colorectal metastases and a CRS of 0 should undergo conventional imaging alone prior to surgical exploration.