PURPOSE: To evaluate the correlation between change in FDG uptake before and after chemotherapy in hepatic metastases of colorectal carcinoma (HCRC) and a histopathologic tumor regression grade (TRG). METHODS: In patients with HCRC, PET/CT data prior to hepatic surgery were retrospectively analyzed under an IRB waiver. The maximum standard uptake value (SUV(max)) was measured before and after chemotherapy. The relative change of FDG activity in the identified lesions was calculated (dSUV). Histopathological specimens of resected metastases were graded on a 5-score TRG scale. A TRG of 1-3 was rated as a responding to therapy, whereas TRG 4-5 were regarded as non-responding lesions. RESULTS: 31 lesions were identified in 23 patients. Mean SUV(max) before and after therapy was 6.9 ± 3.7 and 3.5 ± 1.8, respectively. The area under the receiver operator characteristic curve revealed a conclusive correlation between TRG and dSUV (AUC 0.773; 95 % confidence interval 0.599-0.946) with a cut off at 41 % decrease in FDG activity yielding a sensitivity and specificity of 72 and 75 %, respectively. CONCLUSION: A relative change in FDG activity (dSUV) of more than 41 % decrease correlated significantly with histopathological tumor regression and might be a prognostic tool for response to chemotherapy in HCRC.