STUDY DESIGN: To analyze the relationship between the adjacent superior segment disease and facet joint violations after lumbar fusion. OBJECTIVES: We retrospectively analyzed the relationship between the adjacent superior segment disease and facet joint violations after lumbar fusion. SUMMARY OF LITERATURE REVIEW: Among numerous literatures regarding adjacent superior segment disease, there is no analysis concerning the relationship between adjacent superior segment disease and facet joint violations after lumbar fusion. MATERIALS AND METHODS: We reviewed 2056 patients who underwent lumbar fusion, between March 2004 and April 2009. Analysis was performed for 79 (3.8%) of the 2056 patients with adjacent superior segment disease and needed a second operation. A facet joint was considered as 3 types of violations with computed tomography scans if any of the following situations were encountered: pedicle screw clearly within the facet joint; pedicle screw head clearly within the facet joint; and pedicle screw and/or screw head within 1mm from or abutting the facet joint, without clear joint involvement. RESULTS: The incidence of the violations was 45% (36/79) of all patients and 28% (44/158) of all screws. The incidence of L4-5 facet joint violations was 35% (28/79) of patients with adjacent superior segment disease, statistically. CONCLUSIONS: Facet joint violations were observed in patients with the adjacent superior segment disease after posterolateral lumbar fusion. Because L3-4 facet joint violations increased when L4-5 fusion was done, more care should be taken to avoid facet joint violations when the surgeon is considered for insertion of the pedicle screws at L4-5.