Aim: Body composition, especially the mass of adipose tissue, affects the risk of developing the metabolic and cardiovascular diseases as well as some cancers. The aim of this study was to determine the relationship between the body composition of adults and their Body Mass Index. Material/Methods: The study involved 120 subjects (69 women and 51 men) aged 19 – 66 (30.55 ± 10.41). The recruited subjects were assigned to three subgroups: with normal body weight, overweight and obesity, depending on the BMI value. There were 40 subjects in each subgroup, including 23 women and 17 men. Besides the measurements of height and body mass, the SECA mBCA515 analyser was applied for the body composition analysis using the bioelectric impedance method. Results: A significant association was found between the BMI index and fat mass, lean mass and muscle mass, both in the whole group and after taking into account the sex. The correlation coefficient R range was from –0.88 to 0.97. The incidence of obesity in the studied group according to body fat content criteria (>25% for men and >30% for women) was 57%, while according to BMI criteria –33%. BMI cut-off points for obesity were 27.8, 26.4 and 26.4 kg/m2 for men, women and for all, respectively. Conclusions: In epidemiological studies, to identify obese people, body fat should also be taken into account in addition to BMI. If obesity is understood as excess fat, and not excess weight, the cut-off points for BMI-based obesity should be lowered.