The purpose of this study was to clinically evaluate experimental peri-implant breakdown. Hydroxyapatite-coated, titanium plasma-sprayed, and machined titanium-alloy surfaces were investigated. Eighty-four implants were placed in 14 beagle dogs. Pocket proving depths and clinical attachment level and mobility measurements were made. Dogs were sacrificed at 3 and 6 months. All experimental implants showed a significant loss in clinical attachment level (P .05). Increased pocket probing depths for experimental implants occurred during the first 2 months, after which a plateau was reached. At the 3- and 6-month evaluation, pocket probing depths at experimental implants were significantly increased (P .05). No differences among the three implant types were noted for clinical ttachment levels and pocket probing depths. In general, greater mobility was found with the titanium-alloy implants than with hydroxyapaptite-coated and titanium plasma-sprayed implants (P .025). In addition, mobility measurements were significantly greater for experimental titanium-alloy implants during the frist 3 months (P .05). Clinical attachment level measurements were most sensitive to peri-implant status. All implants were equally susceptible to ligature-induced peri-implant breakdown. Consequently, meticulous oral hygiene and regular maintenance care are prerequisites for successful implantology.