Background:Controversy exists in the literature about whether facet effusions and other degeneration parameters are associated with instability.Objective:To assess the association between facet effusions and other lumbar degeneration parameters and segmental instability.Material and Methods:In this study, 207 L4–L5 and L5–S1 levels in 104 patients were assessed. We divided the spinal levels into two groups: the small facet effusions (SFE) group in whom facet effusions were <1.5 mm or non-effusions were found, and the large facet effusions (LFE) group in whom they were ≥1.5 mm. The association between other degeneration parameters and instability was also assessed, such as disc degeneration, Modic changes (MC), spondylolisthesis, facet orientation and tropism, facet subchondral sclerosis, and facet cartilage degeneration. Furthermore, we subdivided the levels into subgroups to evaluate the association of LFE and instability within each one.Results:The main analysis comparing the presence of instability in SFE and LFE groups showed a non-statistically significant association between LFE and instability. The presence of MC type 1 and the existence of L4–L5 spondylolisthesis had a statistically significant association with instability. In the subset of 43 levels with L4–L5 degenerative spondylolisthesis, the presence of LFE and the existence of MC type 1 reached a significant association with instability.Conclusion:The presence of LFE and/or MC type 1 may act as red flags in patients with L4–L5 degenerative spondylolisthesis to suspect segmental instability.