Radiographic outcome and complications of Harrington-DTT (H) and Cotrel-Dubousset (CD) instrumentation of idiopathic adolescent thoracic scoliosis were compared retrospectively. The patient material consists of 55 consecutive patients in the H group and 52 consecutive patients in the CD group. The mean age at operation was 15 ± 2 years (range, 11–19 years) in both groups. The follow-up interval was 43 ± 18 months (range, 17–91 months) in the H group and 28 ± 11 months (range, ten to 53 months) in the CD group (p = 0.0001). The preoperative radiographic measurements (Cobb angle of primary and secondary curve, apical rotation, thoracic kyphosis, lumbar lordosis) were fully comparable in both groups. The mean correction of the primary curve at the follow-up evaluation was 47% in both groups. Apical rotation of the primary curve remained almost unchanged in both groups. Rotation of the secondary curve increased significantly in the CD group. Thoracic kyphosis was unchanged in the CD group but decreased in the H group. Spinal balance worsened in 29% of the H cases and in 44% of the CD cases. There were no neurologic complications in either of the groups. Intraoperative lamina fractures (four cases) and postoperative hook dislodge-ment (five cases) occurred only in the CD group. One distraction rod breakage occurred in the H group. Three reoperations were performed in the H group, nine in the CD group.