Changes in bone structure like chondronecrosis and osteonecrosis occurred in a group of adolescent athletes especially gymnasts. To determine the etiology calcium, inorganic phosphorus, alkaline phosphatase activity and levels of 25-OH-Vit D were investigated. Single low calcium values were present in nearly the half of the athletes. In 37% the vitamin-D-levels were depressed. Such results were found in athletes with and without changes in bone structure. A vitamin D supplementation is recommended if low vitamin D levels are found. Vitamin D deficiency is not the only reason for the disturbances of the bone. Other factors are to take in consideration like nutrition, delayed bone maturation and its consequences, and the mechanical load.