Abstract: To clarify the mechanism of osteoporosis in adolescent idiopathic scoliosis (AIS), we investigated radiological and histological changes in the cervical vertebrae of a chicken thoracic scoliosis model. Forty newly hatched broiler chicks were randomly divided into four equal groups: sham-operated chickens serving as control (CON), pinealectomized chickens (PNX), sham-operated (CON + MLT) and pinealectomized chickens (PNX + MLT) that received intraperitoneal administration of melatonin. Pinealectomy was performed at the age of 3 days, and the chickens were killed at 2 months of age. Postmortem X-rays were examined for the presence of scoliosis, and micro-computed tomography (micro-CT) images were taken to evaluate the microstructure of the cervical vertebrae. Histological specimens of the scanned cervical vertebra were prepared, and a midsagittal section was stained with hematoxylin and eosin and tartrate-resistant acid phosphatase to evaluate the numbers of osteoblasts and osteoclasts, respectively. Scoliosis developed at the thoracic spine in all chickens of the PNX and in two of the PNX + MLT group. Micro-CT data revealed that chickens in the PNX group had a greater degree of generalized osteoporosis compared with the other birds. The number of osteoblasts was significantly decreased in the PNX group, while no significant difference was observed among chickens in the numbers of osteoclasts. Our results suggest that melatonin deficiency reduces osteoblast proliferation and leads to the development of scoliosis and osteoporosis. The restoration of melatonin prevented the development of scoliosis and osteoporosis, indicating that melatonin levels may be crucial to the development of deformity and osteoporosis in AIS.