The purpose of the present study was to evaluate the effect of autogenous bone graft particles and expanded polytetrafluoroethylene (ePTFE) membrane in the treatment of peri-implantitis with stereologic and histologic methods. Clinical and radiographic findings are reported elsewhere. Experimental peri-implantitis with a bone loss of 4-6 mm was established during 14-22 months around 64 implants with a titanium plasma-sprayed (TPS) surface in eight cynomolgus monkeys (Macaca fascicularis). The defects were treated with autogenous bone+membrane (B+M), autogenous bone (B), membrane (M), or a conventional flap procedure (control) (C). The animals were killed 6 months after surgery. Healthy peri-implant tissue was established irrespective of the applied treatment procedure. However, the amount of bone (autogenous bone graft particles and regenerated bone) and re-osseointegration were significantly higher in defects treated with B+M as compared with the three other treatment modalities. A mean bone-to-implant contact of 45% was estimated within defects treated with B+M. The corresponding values for the B, M, and C groups were 22, 21, and 14%. The present study therefore demonstrates that autogenous bone graft particles covered by an ePTFE membrane is a useful surgical treatment procedure of experimental peri-implantitis around implants with a TPS surface in cynomolgus monkeys. Obviously, there is a background for long-term evaluation in humans.