Positron emission tomography (PET) was applied to evaluate therapeutic effects in patients with advanced head and neck cancer for use in monitoring therapy. In 18 patients with histologically proven head and neck cancer, PET studies with 330-440 MBq 18F-fluorodeoxyglucose (FDG) were performed prior to the first chemotherapeutic cycle with cisplatin and 5-FU. A second examination after the first chemotherapeutic cycle was performed in 11 patients. Tumor or lymph node volumes were determined from CT slices and the growth rate was calculated assuming an exponential function. Uptake in a region of interest was used for the quantitative evaluation of the PET images after standardization to injected dose and body weight. FDG data were available for 6 tumors and 10 metastases, volumetric data for 5 tumors and 7 metastases. One lesion showed an increase, seven a decrease in FDG uptake and eight lesions remained unchanged. Multiple lymph nodes in the same patient showed different baseline metabolisms 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 for treatment planning in patients undergoing systemic chemotherapy.