Results of PET studies using 18-fluorine deoxyglucose (18-FDG) in patients with large-size masses (51) are discussed. Histologically-confirmed prostate cancer was diagnosed in 36 (70.5%), benign hyperplasia--12 (23.5%) and chronic prostatitis--3(6%). 18FDG PET was conducted as whole body irradiation. Our results established its high predictive significance in identifying the scope of tumor involvement. However, the latter's potential is limited in diagnosis of primary tumor node due to low rate of glycolysis in it.