The aim of our investigation was to determine the urine specific gravity in football players at rest and its variations in dependence on the lean body mass and total water content in the body. Fifty-seven male high qualified football players participated in the investigation voluntary. They were 22 ± 3 year olds, the mean height was 182 ± 6 cm, weight – 76.7 ± 8.0 kg and body mass index – 23.2 ± 1.7 kg/m2. The measurements are performed before training, two hours after meal and 30 minutes after voiding. The measurements were performed at warm weather conditions: the air temperature was 24 - 26˚C, relative wetness of air was 60 – 70%. Body hydration degree is determined using urine refractometry method by measuring of rest urine specific gravity before training by digital urine refractometer PAL-10S (Atago, ASV). The athletes’ body mass composition was estimated by bioelectrical impedance analysis method using body mass composition analyzer BC-545 (Tanita, Japan). Approximately half of all football players (49%) were hypohydrated before training (their urine specific gravity was greater than 1.020): 44% moderately (urine specific gravity was 1.020 – 1.030) and 5% seriously (urine specific gravity exceeded 1.030)) hypohydrated. This means that water and mineral salts uptake strategy and education of coaches and football players about uptake of liquids before, during and after training is very important. The correlation between the lean body mass and urine specific gravity is not significant (r = 0.09; p > 0.05). Therefore larger amount of metabolites in urine due to greater skeletal muscle mass in athletes does not play important role in urine specific gravity determination, but it depends on the water uptake. The significant relationship between the total water content in the body and urine specific gravity is not detected (r = 0.002; p > 0.05). This means that bioelectrical impedance analysis method is not useful in the body hydration degree changes estimation.