The uptake of 18F-Deoxyglucose (FDG) was studied in vivo in patients with head and neck tumors with positron emission tomography (PET) in relation to the proliferation rate of these tumors. The quantitative analysis of the radioactivity concentrations revealed two groups, showing a high or a lower FDG uptake pattern. In both groups the FDG uptake and the proliferation rate were correlated with a flat slope of the regression function. It is suggested that these differences in uptake in histologically identical tumor populations may correspond to differences at the molecular level, e.g. differences in the amount of the glucose carrier, perhaps caused by activated oncogenes. Furthermore PET was applied to evaluate therapeutic effects in patients with advanced head and neck cancer. We found that multiple lymph node metastases in the same patient can show a different baseline metabolism and also different changes following therapy. Tumors were more sensitive to therapy than lymph node metastases. The growth rate and the change in FDG uptake were highly correlated with different regression functions for tumors and lymph node metastases. These data demonstrate that PET with FDG can be used to assess early chemotherapeutic effects. The information gained with PET can be included in the treatment planning in patients undergoing systemic chemotherapy.