Background: Thoracolumbar junction (TLJ) is the transitional area between the lower thoracic spine and the upper lumbar spine. Vertebral compression fractures and proximal junctional kyphosis following spine surgery often occur in this area. Therefore, the study of development and mechanisms of thoracolumbar junctional degeneration is important for planning surgical management. This study aimed to review radiological parameters of thoracolumbar junctional degenerative kyphosis (TLJDK) in patients with lumbar degenerative kyphosis and to analyze compensatory mechanisms of sagittal balance. Methods: From January 2016 to March 2017, patients with lumbar degenerative kyphosis were enrolled in this radiographic study. Patients were divided into two groups according to thoracolumbar junctional angle (TLJA): the non?TLJDK (NTLJDK) group (TLJA <10°) and the TLJDK group (TLJA≥10°). Complete spinopelvic radiographic parameters were analyzed and compared between two groups. Pearson or Spearman correlation coefficients and independent two?sample t?test or Mann?Whitney U?test were used. Results: Atotal of 77 patients with symptomatic sagittal imbalance due to lumbar degenerative kyphosis were enrolled in this study. There were 34 patients in NTLJDK group (TLJA <10°) and 43 patients in TLJDK group (TLJA ≥10°). The median angle of lumbar lordosis (LL) in the NTLJDK or TLJDK groups was 23.40° (18.50°, 29.48°) or 19.50° (13.30°, 24.55°), respectively. The median TLJAs in all patients and both groups were ?11.20° (?14.60°, ?4.80°), ?3.70° (?7.53°, ?1.73°), and ?14.30° (?17.45°, ?13.00°), respectively. In the NTLJDK group, LLwas correlated with thoracic kyphosis (TK; r = ?0.400, P = 0.019), sacral slope (SS; r = 0.681, P < 0.001), and C7?sagittal vertical axis (r = ?0.402, P = 0.018). In the TLJDK group, LL was correlated with TK (r = ?0.345, P = 0.024), SS (r = 0.595, P < 0.001), and pelvic tilt (r = ?0.363, P = 0.017). There were significant differences in LL, TLJA, TK, SS, and pelvic incidence (PI) between two groups. Conclusions:Although TLJDK is common in patients with lumbar degenerative kyphosis, it might be generated by special characteristics of morphology and biomechanics of the TLJ. To maintain sagittal balance, pelvis back tilt might be more important in patients with TLJDK, whereas thoracic curve changes might be more important in patients without TLJDK.