Purpose: The effect of two levels of prosthesis misfit on prosthetic screw stability was evaluated. Materials and Methods: Two levels of vertical discrepancies—100 and 175 µm—were introduced between an implant-supported complete denture and the terminal abutment. An implant-supported complete denture without vertical discrepancy served as a control. Cyclic load was delivered vertically on the cantilever portion of the prosthesis next to the terminal abutment for 48 hours for each trial. A total of seven sets of new screws were tested for each level of fit. Results: The results revealed significant prosthetic screw instability at both the 100- and 175-µm levels of discrepancy. Conclusion: Vertical discrepancies of 100 and 175 µm introduced between an implant-supported fixed complete denture and the terminal abutment resulted in significant prosthetic screw instability. Int J Prosthodont 2002;15:38–42.