The breakdown of bone around oral implants following excessive occlusal load or plaque accumulation was evaluated in monkeys. 5 screw type implants of pure titanium (Astra) were inserted in the mandible of 4 monkeys (Macaca Fascicularis). 2 implants were placed in each of the lateral segments and 1 in the frontal area. Each monkey was provided with 2 cemented splints covering the premolars and molars in the right and left side of the maxilla, respectively. 6 months after insertion of the fixtures, a fixed partial prosthesis was mounted on the 2 implants in one of the lateral segments. The prosthesis was in supra-occlusal contact with the antagonizing splint. Each prosthesis was replaced during the course of the experiment. The renewed prosthesis caused a lateral displacement of the mandible during occlusion, and therefore resulted in a lateral rather than axial excessive occlusal load. Implants retaining the prosthesis were brushed 1 x a week and subgingival cleaning was performed 1 x a month. The remaining implants were never cleaned and, additionally, a cotton cord was placed passively around each of these to promote plaque accumulation. 5 out of 8 implants with excessive occlusal load lost osseointegration (mobility and peri-implant radiolucency). The loss of osseointegration was observed 4.5 months to 15.5 months after the occlusal overload was commenced. None of the implants with plaque accumulation lost osseointegration, although an average loss of 1.8 mm in the radiographic bone level was assessed after 18 months.