Study Design. A retrospective outcome study was conducted. Objective. To analyse the clinical and radiographic outcome of surgical treatment for idiopathic scoliosis in patients treated at the age of 20 years or older. Summary of Bavkground Data. Idiopathic scoliosis continues to pose problems in many patients after adolescence. Surgical treatment of idiopathic scoliosis in adults still is a challenging subject, but no outcome analysis in terms of different age groups had been performed. Methods. In this study, 58 adult patients with idiopathic scoliosis who underwent surgery using Cotrel-Dubousset instrumentation were followed up for 2 to 7 years, and the results were studied clinically and radiographically. Results. The prevalence of pain theat requires specific treatemt decreased from 16% to 5%, and the pain improvement was reliably obtained in the older patients. Radiographically, the mean rate of Cobb angle correction was 53% for both the thoracic and lumbar curves, whereas it decreased age dependently, especially in relation to the thoracic curve. The sagittal plane correction generally was satisfactory in patients younger than 50 years, but appropriate lumbar lordosis was difficult to maintain after the age of 50 years. Frontal and sagittal balance often was difficult to achieve in patients older than 50 years. Conclusion. This study clarified specific characteristics and problems in the surgical treatment of adult idiopathic scoliosis according to different age groups. Although pain was the most important indication to surgery in patients older than 40 years, it was improved reliably by surgery in most fo these patients. On the other hand, the radiographic results tended to be less satisfactory in the older patients in and age-dependent manner. These results provide useful information concerning the indications and strategies of adult scoliosis surgery.