Breakdown of bone around oral implants following occlusal overload or plaque accumulation was evaluated in monkeys. 5 screw-type implants of pure titanium (Astra Tech) were inserted in the mandible of 4 monkeys (Macaca Fascicularis). 6 months after insertion of the implants a fixed partial prosthesis was mounted on the 2 implants in 1 of the lateral segments. The prosthesis was in supra-occlusal contact with an antagonizing splint and caused a lateral directed excessive occlusal load (overload). Implants retaining the prosthesis were brushed 1 x week and subgingival cleaning was performed 1 x month. The remaining implants were never cleaned and, additionally, a cotton cord was placed around the abutments of these implants to promote plaque accumulation. 6 out of 8 implants with occlusal overload became loose. 2 of these were lost, whereas the remaining 4 were retained in the jaws. After 18 months of occlusal load or plaque accumulation, the monkeys were sacrificed. Tissue blocks with the implants were infiltrated and embedded in acrylic resin. Approximately 50 microns thick sections of the implants and surrounding tissues were made. All implants with plaque accumulation were osseointegrated, but exhibited an average histologic marginal bone loss of 2.4 mm (range: 0.8-4.0 mm). Of the 6 implants with occlusal overload available for histologic analysis, 2 implants in 1 monkey had lost osseointegration completely and 2 other implants were osseointegrated in the apical part only, whereas the remaining 2 were still osseointegrated but exhibited a bone loss of 1.8-1.9 mm.