Nonresectable colorectal cancer metastases in the liver respond to chemotherapy in 20-25% only. Early identification of nonresponders might allow the use of other regimens. In a limited feasibility study, it should be determined whether (a) a single high-dose chemotherapy application has an early effect on glucose-utilization, detectable and quantitatable by noninvasive positron emission tomography using [18F]-Fluoro-deoxyglucose (FDG-PET) and (b) assess its value as a predictor of the final therapeutic outcome. A total of 10 patients with documented nonresectable liver metastases of a colorectal cancer were studied by FDG-PET, prior and 72 h after a single infusion of 5-Fluorouracil and Folinic acid (5-FU/FA). Glucose utilization was quantitated by determination of standard-uptake values and correlated with final therapy outcome following completion of the anticipated therapy cycle. Patients were followed up for at least 6 months. All metastases responding to therapy (n = 6) exerted a statistically significant decrease of FDG uptake (-22+/-10%), metastases (n = 2) showing a short-term effect (duration of tumor reduction <3 months) had a slightly diminished, and progressing metastases (n = 3) an enhanced FDG uptake (13+/-17%). Our preliminary data indicate that acute changes of glucose utilization-as detected by FDG-PET-following a single application of chemotherapy, seems to be indicative for the final therapeutic outcome, at least in liver metastases of colorectal cancer.