Ventral derotation spondylodesis, according to Zielke, achieves good results in operative treatment of idiopathic thoracic scolioses. Corrections of scoliotic major and secondary curve as well as derotation of the spine are reliably performed. The high rate of rod fractures with subsequent correction loss as well as a proportionate kyphogenic effect represents a problem. By keeping to the correcting principle, anterior double-rod instrumentation (Halm-Zielke Instrumentation) is to be stable in a similar way as posterior double-rod systems. Thus, it is done to facilitate brace-free postoperative care and to prevent excessive kyphotic pattern of the spine. In this prospective study, we retrospectively collected data. We performed radiological follow-up of two groups of patients with idiopathic thoracic scoliosis (King II, III and IV) undergoing an operation with posterior approach (USS instrumentation, posterior group, n=104) in 1997 and 1998 or being corrected with an anterior fusion (Halm-Zielke instrumentation, anterior group, n=37) between 2000 and 2001. Mean age of all patients for operation was 15+/-4 years. Follow-up was performed after 4+/-2 years on average. Preoperative measurements of the major and secondary curve, the lateral profile, rotation and frontal balance (C7 to S1) did not show any significant differences apart from a more severe scoliotic curve in the lumbar spine for the anterior group with appropriately higher lumbar rotation. During follow-up we noticed similar corrections of the thoracic major and lumbar curve in both groups ranging from 49 to 56%. In case of hypokyphotic (T4-T12or=40 degrees ) scolioses were flattened by posterior spinal fusion; the effect of anterior spinal fusion was not significant. Correction of thoracic and lumbar rotation in the anterior group by 37 or 30% was more significant than in the posterior group by 27 or 20%. There was no impact of anterior technique on the balance of the spine whereas the latter shifted by an average of 7 mm to the left in the posterior group. The number of fused segments was significantly smaller in the anterior group with 7+/-1 vertebral bodies (posterior, 11+/-1 vertebral bodies). Rates of complication were identical with 11 or 12% in both groups during follow-up. Anterior and posterior double-rod instrumentations result in comparable corrections for idiopathic thoracic scoliosis of the major and secondary curve. In case of posterior technique, however, four vertebral bodies less were integrated in spondylodesis on average. Balance of the spine did not change after anterior spondylodesis; however, it declined by using the posterior technique. Augmentation of the anterior threaded rod combined with a solid second rod significantly decreases the rate of implant breakages and reliably reduces consecutive correction losses.