Objective. The objective of this study is to determine the impact of postoperative spinopelvic parameters on the development of adjacent segment instability after single-level lumbar fusion. Material and methods. A total of 116 patients with degenerative spine conditions after lumbar fusion were enrolled in this study and subdivided into two groups. Group I consisted of 24 patients with signs of adjacent segment instability; Group II included 92 patients without signs of instability. The minimal follow-up period was 24 months. Results. The mean postoperative lumbar lordotic (LL) angle in both groups was within the normal range (–60.9±12); no statistically significant intergroup differences were revealed (56.6±12.1 and 58.4±11.2 for Groups I and II, respectively; p=0.314). In Group I patients, the mean pelvic incidence (PI) angle differed significantly from the mean PI values in Group II patients (70.4±7.6 and 53.2±8.4, respectively; p=0.006) and from the normal PI values (51.9±10). Therefore, the mean difference between PI and LL (PI—LL) angles in the Group I patients was significantly higher than in Group II (16.2±5.4 and 4.8±8.6, respectively; p=0.004). Significant PI—LL mismatch (PI—LL ≥10°) was observed in 22 (91.7%) patients in Group I and in 11 (11.95%) patients in Group II. According to regression analysis data, the PI—LL mismatch was identified as a risk factor for adjacent segment instability; the odds ratio =4.2; 95% confidence interval 1.46—12.25; and p=0.007. Conclusion. Patients with the high PI value and low LL value have a significantly higher risk of adjacent segment instability after short-segment spinal fusion.