To evaluate the relationship between PET using 18F-fluorodeoxyglucose (FDG) and pathological findings and to compare indices obtained by FDG-PET, FDG-PET was performed in 23 patients with untreated malignant lymphoma. Three indices obtained by FDG-PET, tumor-to-normal contrast ratio (TCR), distribution absorption ratio (DAR), k1k2/(k2 + k3), correlated with proliferative activity which was pathologically estimated both by mitotic count and by proportion of cells in all phases of the cell cycle. The relationship did not significantly change according to which of the three indices was chosen. FDG-PET, which shows the proliferative activity of tumors, is considered to be a useful method for managing tumors.