OBJECTIVES: Evaluate the cost of care for patients undergoing posterolateral spinal fusion (PLF) between on-label Si-CaP use and off-label rhBMP-2 use. It is hypothesized that Si-CaP patients will have lower Total patient costs (cost of entire inpatient hospitalization), lower supply costs, and lower Operating Room costs. METHODS: Patients over age 18 having a PLF procedure (ICD9 codes 81.62, 81.63, or 81.64) and receiving Si-CaP or rhBMP-2 on day of procedure between January, 2006 and December 31, 2010 were selected. Comorbidities were identified using a modified Charlson method. Patients with 9 or more levels fused were excluded. Univariate group comparisons were made using chi-square and student t-tests. Multivariate linear models were developed using both normal and log transformation. Discharges with costs less than $1,000 or greater than $500,000 were excluded in the normal multivariate models but were included in the log transformed models. RESULTS: A total of 60,260 patients were initially identified. After applying exclusion criteria 59,229 patients were available for analysis (98.3%). Univariate comparisons of Total Cost as well as OR and Supply costs were significantly lower for the Si-CaP cohort (p <0.05). Adjusting for patient and hospital covariates also showed statistically significant lower estimated mean costs for Si-CaP for all outcomes (p <0.001). Differences in estimated means for Total Costs were lower for Si-CaP $8931 and $7582 using normal and log-transformed values respectively. Estimated means for Supply Costs showed similar results of $7255 and $5274 for normal and log-transformed values respectively. Operating Room Costs were also similar, $961 and $809. CONCLUSIONS: After adjusting for patient and hospital covariates Si-CaP patients had significantly lower Total Patient Costs. While Supply Costs showed similar results they do not fully explain the reduction in Total Cost suggesting that there are other cost advantages besides product acquisition cost. Further research should consider cost of complications and readmission.