Purpose: The purpose of the present investigation was to clinically assess and compare crestal bone changes, over a 5-year period, around external-hexagon wide-diameter implants restored with either matching wide-diameter prosthetic components or with platform-switched prosthetic components. Materials and Methods: During the years 2000 to 2002 all patients who received a single 5-mm-diameter implant with an external hexagon in a private office setting were included in this study. All implants were placed in the posterior areas of the jaws. Maxillary left molars (group A1) and mandibular right molars (group A2) were restored with matching wide-diameter prosthetic components; maxillary right molars (group B1) and mandibular left molars (group B2) were restored with platform-switched prosthetic components. Marginal bone resorption was measured via intraoral radiographs each year after abutment and crown insertion. Statistical analyses were used to determine whether there was a significant difference in marginal bone levels with respect to the width of prosthetic components used. Results: In all, 182 single 5-mm-diameter implants were placed in 144 patients and all implants survived. Eighty-five implants were restored with matching wide-diameter prosthetic components (group A), and 97 implants were restored with platform-switched prosthetic components (group B). A significant difference in marginal bone levels was found between group A and group B implants after 1 year. The mean marginal bone resorption was 0.9 mm (SD 0.3 mm) for group A implants and 0.6 mm (SD 0.2 mm) for group B implants. Marginal bone resorption observed at the second, third, fourth, and fifth years after abutment and crown insertion did not show any significant change. Conclusion: Statistically significant differences in marginal bone loss were observed between study groups. The 85 implants restored with matching wide-diameter prosthetic components showed more bone loss than the 97 implants restored with platform-switched prosthetic components. Keywords: dental implants, implant diameter, platform switching