METHODS Prospective cohort study. OBJECTIVE In this study, we investigated the frequency of vertebral endplate Modic changes (MCs) and their effects on surgical outcomes in patients with unstable lumbar spines. BACKGROUND Signal changes in endplates have been classified into three types by Modic. The prognostic role of MCs has been investigated in various spinal disorders. METHODS A series of 70 patients with clinical and radiographic unstable lumbar spine were included in the study. Endplate signal intensity was determined according to Modic classification. All patients underwent instrumented posterolateral fusion. Functional evaluation was made using the visual analog scale (VAS) and Oswestry disability index (ODI). RESULTS Eighteen patients (26%) had normal endplate intensity, 31 patients (44%) had MC type I, 20 patients (28%) had MC type II, and one patient (1.4%) had MC type III. Pain level VAS and ODI decreased significantly from the preoperative evaluation to the six-month and one-year postoperative evaluations. The surgical outcome (VAS and ODI) was not significantly different between the various types of MC. CONCLUSIONS Posterolateral fusion is an effective treatment in patients with unstable lumbar spines. MC do not have a significant effect on the surgical outcome of these patients.