INTRODUCTION: In the last few decades, obesity has reached epidemic proportions worldwide, representing a significant medical and public health problem. The most vulnerable are patients with morbid obesity which puts them at a high cardiovascular risk. Body mass index is not the most accurate indicator of body composition. ----- AIM: To determine whether there is a difference in body composition of subjects with third degree obesity in comparison to healthy normal weight subjects, i.e. to evaluate peculiarities in the body composition of morbidly obese subjects. We also wanted to investigate the association of body composition with diabetes and arterial hypertension. ----- METHODS: This study included 17 obese subjects with body mass index > 40kg/m2, without secondary causes of obesity. Control group consisted of 14 healthy normal weight subjects. Body composition was analyzed by the bioelectrical impedance analysis method (BIA); analyzed parameters were body fat mass, fat percentage, lean body mass and lean mass percentage. ----- RESULTS: In comparison to healthy normal weight subjects obese group was found to have significantly higher body fat mass and percentage of body fat, as well as lean body mass (p<0,001). Obese subjects had significantly lower lean mass percentage (p<0,001). There was also a strong correlation between body composition and the existing diagnosis of hypertension (p<0, 05) and a moderate correlation with the diabetes of diagnosis (p<0, 05). ----- CONCLUSION: BIA has shown a deficit of lean body mass, which points to sarcopenia in morbidly obese patients (high fat percentage and low muscle mass percentage) and cannot be detected by measurement of body mass index. Components of metabolic syndrome, hypertension and diabetes, correlated with body composition elements of morbidly obese people.