There is a lack of studies concerning hydration status of young athletes exercising in the heat. PURPOSE: To assess pre-exercise hydration status in young soccer players during a summer sports camp and to evaluate body water balance after soccer training sessions. METHODS: Initial hydration status was assessed in 107 young male soccer players (age: 11-16 years) during the 2nd day of the camp. Seventy two athletes agreed to be monitored during two more training sessions (3rd and 5th day of the camp) in order to calculate dehydration via changes in body weight, while water drinking was allowed ad libitum. Hydration status was assessed via urine specific gravity (USG), urine color and changes in total body weight. Mean environmental temperature and humidity were 27.2±2°C and 57±9%, respectively. RESULTS: According to USG values, 95 out of 107 of the players were hypohydrated (USG≥1.020), prior to practice. The prevalence of dehydration observed was maintained in both days, with 95.8% and 97.2% of the players being dehydrated, after the training sessions in the 3rd and 5th day, respectively. Despite fluid availability, 54 out of the 66 (81.8%) dehydrated players reduced their body weight (-0.35±0.04 kg) as a response to training, while 74.6% (47 out of the 63) reduced further their body weight (-0.22±0.03 kg) after training in the 5th day. CONCLUSION: Approximately 90% of the young soccer players who began exercising under warm weather conditions were hypohydrated; while drinking ad libitum during practice, did not prevent further dehydration in already dehydrated players.nhttp://www.ncbi.nlm.nih.gov/pubmed/23166200n