OBJECTIVES To test whether GBR with an L-shaped soft-block bone substitute and particulate bone substitute differs from GBR with particulate bone substitute as regards the volume stability of the augmented region during flap closure. MATERIALS AND METHODS Twenty peri-implant box-shaped bone defects were created in 10 pig mandibles. Every bone defect was augmented with each of the following two GBR procedures in turn: control group - particulate xenograft applied buccally + collagen membrane + pins; test group - particulate xenograft applied buccally + L-shaped soft-block xenograft applied buccally and occlusally + collagen membrane + pins. Cone-beam computed tomography scans were obtained before and after wound closure. The horizontal thickness (HT) of the augmented region (bone substitute + membrane) was assessed at the implant shoulder (HT0 mm ) and at 1 mm to 5 mm apical to the implant shoulder (HT1 mm -HT5 mm ). In the test group, the vertical thickness (VT) and 45° thickness (45-T) of the augmented region were measured from the implant shoulder. The changes in HT during flap suturing were calculated as absolute (mm) and relative values (%). Repeated-measures ANOVAs were used for statistical analysis. RESULTS The reduction in HT0 mm was 20.5 ± 23.3% (SD) in the control group and 2.4 ± 9.2% (SD) in the test group (P = 0.014). There were no statistically significant differences in changes in HT1-5 mm between the groups (P > 0.05). In the test group, the reduction in VT amounted to 28.0 ± 11.9% (SD) and the reduction in 45-T amounted to 24.8 ± 10.2% (SD) (P < 0.001). CONCLUSION The addition of an L-shaped soft-block bone substitute to a particulate xenograft, covered by a collagen membrane and fixed with pins, significantly improved the horizontal volume stability of the augmented region during wound closure.