Purpose: To determine if tumor volume, in addition to tumor metabolic activity, can be assessed noninvasively from attenuation-corrected fluorodeoxyglucose (FDG)-PET imaging using a semiautomated method. Methods: CT and FDG-PET scanning was performed in 14 patients, eight with newly diagnosed untreated malignancies, and six patients with progressive non-Hodgkin's lymphoma (NHL). Tumor volume was determined from CT scans by summation of manually drawn regions of interest over tumor. Tumor volume was determined at FDG-PET with a semiautomated method based on quantitation of 18F uptake and thresholding. Results: Mean tumor volume was 187 ± 189 cm3. Tumor volume determined by means of PET and CT was strongly correlated (r = 0.98, P < 0.001, N = 8) in the patients with untreated tumors. Correlation was weaker (r = 0.70, P = 0.006, N = 14) for all patients, mainly due to one previously treated patient with a large disparity between CT and metabolically active tumor volumes at FDG-PET, presumably due to tumor necrosis. Conclusions: Tumor volume determination by FDG-PET was strongly correlated with tumor volumes determined by anatomic imaging with CT. FDG-PET appears comparable to CT in measuring untreated tumor volumes of this size. FDG-PET may be superior to anatomic techniques in assessing metabolically active tumor volume, and warrants further study in this role.