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 probing 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 attachment levels and pocket probing depths. In general, greater mobility was found with the titanium-alloy implants than with hydroxyapatite-coated and titanium plasma-sprayed implants (P < .025). In addition, mobility measurements were significantly greater for experimental titanium-alloy implants during the first 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.