The uptake of 18F-Deoxyglucose (FDG) was studied in vivo in relation to the proliferation rate of human head and neck tumors. Forty-two patients with histologically proven squamous-cell carcinoma of the head and neck and four patients with metastases of head and neck tumors were examined with PET and FDG prior to surgery. In 35 of these patients, a flow cytometric analysis of the DNA content and the proliferation rate was done using one-dimensional flow cytometry rate was done using one-dimensional flow cytometry (DAPI staining). In 17 cases, perfusion studies with 15O-labeled water were performed. Twenty-seven specimens were evaluable by flow cytometry. The analysis of the distribution of the FDG uptake revealed two groups, showing a high and a lower uptake pattern. In both groups the FDG uptake and the proliferation rate were correlated with an r-value of 0.64 and 0.8 respectively. However, the slope of the regression function was flat. No correlation was found between the perfusion and the proliferation rate. 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 oncogenic transformation.