Purpose: To determine whether a collagen scaffold could provide an environment for mesenchymal stem cell (MSC)-related bone repair of critical-size bone defects in rat calvaria. Materials and Methods: Craniotomy defects were created in 28 adult Sprague-Dawley rats. Two additional rats were used as MSC donors by means of femoral bone marrow lavage and culture. The rats were randomly divided into four groups: (1) empty/no graft; (2) collagen scaffold (matrix) + saline; (3) matrix + MSCs; (4) matrix + bone morphogenetic protein. The animals were euthanized 28 days after surgery. Microcomputed tomographic reconstructions were obtained to measure bone fill. The specimens were processed for histologic examination, and the total defect and bone fill areas were measured. Results: Mean bone fill (± standard deviation) of 9.25% ± 10.82%, 19.07% ± 17.38%, 44.21% ± 3.93%, and 66.06% ± 15.08%, respectively, was observed for the four groups; the differences were statistically significant. Bone repair was statistically significant for groups 3 and 4. No significant difference was seen for bone repair between groups 1 and 2 or between groups 3 and 4. Bone formation differed significantly across the four groups. Statistically significant changes in radiodensity were observed between groups 1 and 3, groups 1 and 4, and groups 2 and 4. Significant differences were not observed between groups 1 and 2, groups 2 and 3, or groups 3 and 4. Conclusion: After grafting of adult MSCs adherent within a collagen matrix, repair of bone was significant. Expanded three-dimensional collagen represents a radiolucent, resorbable, biocompatible scaffold that is capable of supporting MSC repair of bone. Keywords: adult mesenchymal stem cell, bone repair, collagen matrix, osteogenesis, rat calvaria model, tissue engineering