Diagnostic procedure of positron emission tomography (PET/CT) is discussed in this paper. (The aim of this paper is to discuss details of a positron emission tomography (PET/CT) diagnostic procedure.) Physical characteristics of the PET/CT scanner, positron decay, electron–positron annihilation process, and specific positron emitting radionuclides are presented. Fusion of PET and CT tomograms is emphasized, as one of the key elements of image quality. Radionuclide discussed in this paper is F-18-FCH (fluorocholine), a positron emitting radionuclide consisting of methylcholine, used as an indicator of increased cell membrane phospholipid synthesis. Two most common F-18-choline PET/CT diagnostic procedures are reviewed in this paper, F-18-FCH PET/CT in prostate cancer patients and F-18-FCH PET/CT in patients with primary hyperparathyroidism. Prostate cancer is the most common malignancy in men and a third cancer-related cause of mortality in the European union. Following initial therapy, biochemical relapse of the disease occurs in 20 – 50 % of patients. The value of the functional nuclear medicine method F-18-FCH PET/CT specifically in patients after primary therapy with suspected biochemical relapse of prostate cancer is indicated in this study. Primary hyperparathyroidism (PHPT) is a common endocrine disorder, with a prevalence of 1 – 7 cases in every 1000 adults. More than 85% of PHPT are caused by a solitary parathyroid gland adenoma, 15% are caused by a multiple parathyroid gland disease (MGD), and a rare cause of PHPT (1%) is a parathyroid gland carcinoma. F-18-FCH PET/CT is increasingly used in the diagnosis of PHPT.