Factors which affected the severity of scoliosis induced by rib resection were examined. Seventy chickens were divided into a rib transection group and a rib resection group with seven subgroups, according to the number of ribs treated and the age at surgery, and examined radiographically until 20 weeks postoperatively. Rib transection group chickens showed fusion at the transected site and no marked scoliosis. In the rib resection group, the more ribs resected and the younger the age at operation, the more severely the scoliosis developed. Bone regeneration at the resection site made the progression of scoliosis milder. These findings indicate that the age at rib resection, the number of ribs that are operated on, and nonunion of the resected sites play important roles in the progression of scoliosis induced by rib resection.