BACKGROUND: Fluordeoxyglucose ((18)FDG) is the most common radiotracer used for PET/CT studies. It enters the cell because of the glucose transporter proteins (GLUTs): 1) erythrocytic membrane, skeletal muscle, lymphocytes, ovaries, breast; 2) pancreas, retina, erythrocytes; 3) adipocytes, ovaries, testis; 4) skeletal muscle, adipocytes, ovaries, myocardium; 5) breast, small intestine, testis, kidney, erythrocytes; 6) spleen, leucocytes, brain; 7) liver; 8) testis, brain; 9) liver, kidney; 10) liver, pancreas; 11) heart, muscle; 12) heart, prostate; 13) brain. We undertook this study to expand the knowledge about physiological uptake. Physiological uptake of (18)FDG was in brain, Waldeyer ring (adenoids, palatine tonsils, lingual tonsils), salivary glands (parotids, submandibular), tongue, vocal cords, cricoarythenoid muscle, thyroid, brown fat (supraclavicular, mediastinal, neck, pericardial fat, around kidney, around great vessels in the thorax, subdiaphragmatic, intercostals, paravertebral), myocardium, breast, thymus, contractive muscles, liver, spleen (similar to the liver), stomach, intestine, kidneys, bladder, uterus, ovaries, testes, bone marrow, esophagus, and atherosclerotic inflammatory plaque. DISCUSSION: False positives were as follows: pneumoniae, tuberculosis, sarcoidosis, cryptococcosis, thrombosis, bronchitis, costochondritis, radiation pneumonitis, misregistration for respiratory movements, catheters, thyroid and adrenal adenomas, osteophytes, fractures, abscess, foreign body, surgical wounds, ostomies, prosthesis, degenerative joint diseases, osteomyelitis, amyloidosis, pancreatitis, myositis, gastritis, colitis, herpes zoster. (18)FDG should be injected 4-6 h after insulin administration because it will be concentrated in the muscles. The brown fat raises its uptake 50% in late images. CONCLUSIONS: It is vital to know the most frequent sites of physiological uptake in the (18)FDG PET/CT studies to identify those regions that occasionally present hypermetabolism but that are not related to neoplastic tumors. This must be taken into consideration in the evaluation of PET/CT studies.