Study Design. A paired comparison was done of the bending flexibility and compression strength of tricortical bone graft and titanium fiber mesh implants in a human cadaver model of posterior lumbar interbody fusion. Objectives. To test the hypothesis that a titanium fiber mesh implant and a tricortical bone graft provide adequate and equal mechanical strength and stability in posterior lumbar interbody fusion constructs. Summary of Background Data. Although studies of posterior lumbar interbody fusion constructs have been performed, the authors are unaware of any study in which the strength and stability of a titanium fiber mesh implant are compared with those of tricortical bone graft for posterior lumbar interbody fusion in the human cadaver lumbar spine. Methods. Changes in neutral zone and range of motion were measured in a bending flexibility test before and after placement of posterior lumbar interbody fusion constructs. Tricortical bone graft and titanium fiber mesh implant construct stability then were compared in a paired analysis. The constructs then were loaded to failure to evaluate construct strength as a function of graft material and bone mineral density. Results. The posterior lumbar interbody fusion procedure produced statistically significant decreases in neutral zone when compared with the intact spine. No statistically significant differences in neutral zone, range of motion, or strength were detected between the two implants. Construct strength correlated strongly with bone mineral density. Conclusions. Posterior lumbar interbody fusion procedures result in equal or improved acute stability for titanium fiber mesh implants and tricortical bone graft implants when used without additional posterior stabilization.