STUDY DESIGN: This is a prospective study to determine whether calcium sulfate(CS) used as a bone graft expander could promote spinal fusion as effectively as autogenous bone graft. OBJECTIVES: To investigate the ability of CS to serve as bone graft substitute when combined in a 1:1 ratio with autogenous graft bone(AGB). SUMMARY OF LITERATURE REVIEW: Autogenous bone is considered the most successful bone graft material and is presently gold standard. Many complications, however, have been reported. Thus, numerous biodegradable osteoconductive ceramic bone graft substitute have received attention as alternative to autogenous bone to reduce the complications. The advantage of a biodegradable graft material is its compatibility with the new bone remodeling process required to attain optimum mechanical strength. MATERIALS AND METHODS: Fifteen patients who had undergone posterolateral spinal fusion with instrumentation using CS mixed with AGB were evaluated. The patients received the autogenous iliac crest graft on one side of the spine and an equivalent volume of autogenous iliac crest/ CS combination on the other side. Thus, the patients serve as their own control. The number of segments fused was 45 segments. The implanted sites were assessed for new bone formation and bony fusion by plain radiography and CT. RESULTS: Of 47 segments fused with CS and AGB, 42 segments (89.4%) were completely fused. In contrast, segments fused with AGB alone, 44 segments (93.6%) were fused. One patient showed nonunion at the both side. Two patients had nonunion at the fused segments with CS and ABG. However, the other side showed complete union. 5 patients who underwent removal of hardware had grossly and histologically complete union. There were no complications related to CS. CONCLUSION: Calcium sulfate appeared to have some potential as a bone graft expender rather than a graft substitute when combined 1:1 ratio with ABG, despite fusion rate by CS mixed with AGB ws lower than AGB alone.