In this consecutive, prospective study, the value of an FDG-PET scan acquired before treatment of liver metastases from colorectal cancer was tested in 54 patients. In 81% of the cases, PET findings were in concordance with CT. In 19% of the cases, the treatment plan was altered as more liver lesions were found by PET than by CT (4 patients), fewer or no liver lesions (3 patients), or extra-hepatic lesions (3 patients). PET used supplementary to CT improves the treatment decision in one fifth of patients with colorectal liver metastases.