The purpose of this study was to evaluate the prognostic value of quantitative dynamic FDG PET studies in patients with metastastic colorectal cancer receiving FOLFOX (fluorouracil, folinic acid and oxaliplatin) chemotherapy. The evaluation includes 28 patients with 55 metastases from primary colorectal cancer. Reference for the FDG studies was the clinical response data, according to the WHO classification. Three response groups were defined: progressive disease (PD), stable disease (SD) and partial response (PR). The FDG studies were accomplished as dynamic series for 60 min. The evaluation of the FDG kinetics was performed using the SUV, and fractal dimension (FD) of the time activity curves based on the box counting procedure (parameter for the inhomogeneity of the tumors). The median SUV as measured in the tumor lesions prior to onset to FOLFOX was 3.15, in comparison with 2.68 SUV after the first cycle and 2.61 SUV after the second cycle. Discriminant analysis (DA) was used for the classification of the data into the 3 categories. Both parameters SUV and FD provided 2 of the 3 "predicted" categories, namely PD and SD. It was possible to correctly classify PR in only 10% of the patients, using the FD of both studies. Generally, DA inclined to misclassify the data towards PD. Even the first PET study was predictive with respect to therapy outcome (96% for PD and 47% for SD using only the baseline SUV). Metastases with a baseline SUV lower than 4.6 did not respond to FOLFOX chemotherapy. The combination of SUV and FD of the first study lead to a correct classification of 93% of PD and 60% of SD. Best results were obtained for the FD of the initial PET study (90% for PD and 75% for SD) as well as for the FD of both studies (77% for PD, 73% for SD, 10% for PR). Quantitative, dynamic FDG-PET should be used preferentially for monitoring patients with metastatic colorectal cancer receiving chemotherapy. Even the first FDG study prior to onset to chemotherapy is predictive for the therapy outcome.