OBJECTIVE: To evaluate the effect of body composition on the quality of the PET / CT images comparing different methods of calculating the application dose (MBq / kg MBq / BMI MBq / cm). METHODS: In 65 patients divided into groups according to body composition depending on body weight (kg), body mass index (BMI) and waist circumference (cm) we did comparison of image quality estimated on the basis of SNR index (engl. Signal-to-noise ratio) of the liver. We also compared the differences in image quality by using various valuation parameters of body composition (body weight, body mass index and waist circumference). RESULTS: In patients with bigger physical composition, image quality determined according to SNR index is worse, regardless of the parameters (body weight, body mass index and waist circumference) used for the calculation. Using the size of the waist gives the same results as the calculation according to body weight and BMI. Patients with body weight above 80 kg, BMI greater than 27 and waist circumference greater than 98 cm need further improve of the quality which is possible by increasing the dose of radiopharmaceutical application or extension of aquisition time. CONCLUSION: In patients with bigger body composition is necessary to improve the image quality which can be achieved by increasing the administered dose or increase the recording time. In the formula which calculats individual doses of radiopharmaceuticals, values of body weight, body mass index and waist circumference can be used.