PURPOSE: Kyphosis of the cervical spine has been reported in patients with adolescent idiopathic scoliosis (AIS). However, few reports have compared sagittal spine alignment of AIS patients with that of the normal population. The purposes of this study were (1) to analyze the characteristics of sagittal alignment, including the cervical spine, in AIS patients with a single thoracic curve (Lenke type 1) compared with the age-matched normal population and (2) to quantify the changes in sagittal alignment of the cervical spine and thoracic kyphosis following posterior spinal fusion. METHODS: In study 1, pre- and postoperative X-ray were measured for the following sagittal plane parameters: lumbar lordosis angle (L1-S1; LL), thoracic kyphosis angle (Th5-12; TK), sacral slope (SS), C7 plumb line (C7PL), cervical lordosis angle (C2-C7 angle; CL), and T1 slope. These measurements were then evaluated with CL and other parameters using spearman rank correlation coefficient between two groups. Comparison was made with the sagittal plane parameters from preoperative 42 AIS (AIS group) with main thoracic curve and 24 normal populations (Control group). In study 2, 38 operative AIS patients had at least 1-year follow-up. These patients (38 AIS patients after the correction surgery) were enrolled. We collected for each patient on pre- and postoperative sagittal plane parameters of X-ray. RESULTS: In study 1, LL and C7PL did not differ significantly between the groups. Although CL was observed in 10 of the 24 patients (41.7 %) in the Control group, the CL was smaller in the AIS group, with 6 of 42 patients (14.3 %). The CL correlated significantly with T1 slope (r = 0.634), C7PL (r = 0.684), and TK (r = 0.311) in the AIS group, and with T1 slope (r = 0.681) and C7PL (r = 0.451) in the Control group. No correlations were observed with respect to the TK. In study 2, the mean CL improved significantly from 7.2° kyphosis preoperatively to 0.1° kyphosis postoperatively. Interestingly, Spearman correlation analysis showed that the postoperative CL correlated significantly with postoperative TK (r = 0.607), postoperative T1 slope (r = 0.701), and postoperative C7PL (r = 0.373). CONCLUSIONS: There were no effects of scoliosis on sagittal spine parameters such as LL and C7PL in AIS patients with a main thoracic curve. Cervical spine alignment was affected by the thoracic deformity in the sagittal plane, as shown by the reduction in the CL after the operation. These findings suggest that TK may be a cause of cervical kyphosis in AIS patients.