ABSTRACT Congenital scoliosis associated with costal malformation is well known. However, there are no reports of idiopathic scoliosis associated with the fusion of the costal arcs in the literature. This report describes a case of idiopathic scoliosis with fusion of the 1st and 2nd costal arches in a female patient and reports changes in the deformity due to scoliosis because of the treatment. The analysis was performed from a morphological and clinical point of view, and by complementary tests. Radiographs and CT scans were obtained and evaluated from the first consultation in 2012 and during the period in which the patient was followed in a private clinic, until the stabilization of the condition, in 2014. The evolution was favorable with the use of a Milwaukee vest and there was improvement of the clinical picture after the skeletal maturity, and the angle of lateral curvature, measured by the Cobb method, initially of 20 degrees, was reduced and stabilized in nine degrees, measure which is currently maintained. The analysis showed concomitant pathologies, and the fusion of costal arches did not influence the spine deformity, since there was regression with the clinical treatment. This evolution leads us to conclude that adolescent idiopathic scoliosis should be imputed as the sole responsible for the clinical picture of the patient, and that the fusion of the costal arches does not interfere with the biomechanics of the spine. Level of Evidence: IV. Type of study: Case series.