This study was peformed to investigate the morbidity of nutritional secondary hyperparathyroidism(NSH) caused by imbalance of Ca and p, and related athletic disease in Cheju pony racehorse. The seventeen horses with clinical signs among 33 NSH affected, administered CaCO$_3$(34 g) and Vita-rinka1(120 g) respectively for 40 days. The results were asd follows; Morbidity of NSR was 33 among 47 horses, and it was caused by the deficiency of Ca in 32 horses. In a case, level of Ca was norm질 although P was high. There was no case of Ca deficiency with P excess. Among 33 NSH affected horses, 13 were subclinical and 20 were clinical types with severe lameness in 6 and transient lameness in 14. Although there was no difference in bone density between transient lameness and normal horses on radiography, among six horses wlth severe lameness two showed hyperplasia at periosteum, one had low density of phalanges and metacarpal bones, and thin cortex. and there with fracture at carpus, nivicular bone and proximal sesamoids. The levels of FECa and FEP were recovered after CaCO$_3$ administration in 2 horses among ten, and after Vita-rinkal in all of seven. The clinical signs were disappeared in slx horses among ten CaCO$_3$ treated, and in five among seven Vita-rinkal treated. There were no differences on radiography in bone density and thickness of cortex on 14 horses with transient lameness. Three horses with severe signs were recovered to normal bone density and thickness of cortex, and there was no significant difference between two groups. In summary, the morbidity of NSH in Cheju pony racehorses was relatively high because of deficiency of Ca. Constant admistration of Ca supplements is desirable to treat and prevent athletic disease development in Cheju racehorses.