Purpose: Soft and hard tissue responses to experimental peri-implantitis around thin sputter hydroxyapatite (HA)-coated implants were evaluated and compared to the responses to implants with other surface treatments. Materials and Methods: Forty-eight dental implants with four different surfaces-machined (M), sandblasted/acid-etched (SA), 1-mu m thin sputter HA-coated (S), and plasma-sprayed HA-coated (P)-were inserted into the mandibles of six beagle dogs. Three months later, experimental peri-implantitis was induced with ligatures to allow plaque accumulation. After a 4-month period of active breakdown, the ligatures were removed, and plaque accumulation continued for 5 additional months (progression period). Radiographic marginal bone levels, probing depths, clinical attachment levels, and modified Gingival Index were evaluated at baseline, after the active breakdown period, and after the progression period. Results: Significant increases in mean probing depths and clinical attachment levels were seen around all implants after active breakdown, but no significant differences were found during the progression period. Radiographic analysis revealed marginal bone loss of 1 to 1.7 mm during the active breakdown period. Additional bone loss occurred during the progression period (M 0.2 mm, SA 0.3 mm, S 0.2 mm, P 0.4 mm). Conclusion: Comparable tissue behavior was demonstrated around dental implants with all four surfaces under peri-implantitis conditions. Thin sputter HA-coated implants possess the favorable osteoconductive properties of calcium phosphate coatings without exacerbating further peri-implant tissue breakdown during the progression of peri-implantitis.