A total of 48 patients with squamous cell carcinomas (SCC) of the head and neck were examined with positron emission tomography (PET) and 18Fluor-deoxyglucose (FDG). In 25 cases the FDG-PET-data were compared with flow cytometric data of the lesions to investigate the value of PET for the noninvasive determination of a tumor's proliferative activity. The quantitative analysis of the radioactivity concentrations revealed two groups of tumors with a different FDG-uptake. A correlation was seen between the proliferative index and the FDG-uptake with an r-value of 0.64 in the lower uptake group and an r-value of 0.81 in the higher uptake group. There was no correlation between any clinicopathological or morphological parameters of the tumors and the FDG-uptake. It is suggested that the different FDG uptake may correspond to differences at the molecular level. The PET-evaluation of therapeutic effects in patients with advanced SCC of the head and neck revealed different posttherapeutic changes of the metabolism in tumors and in lymph node metastases. Tumors were more sensitive to therapy than lymph node metastases. There was a high correlation between the growth rate and the change in FDG uptake with a different regression function for tumors and for lymph nodes. The data demonstrate the usefulness of PET and FDG for the evaluation of early therapeutic effects. Therefore PET can be applied for the treatment planning in patients with malignant tumors of the head and neck.