The purpose of this study was to compare the effect of plaque accumulation in conjunction with or without placement of plaque‐retaining ligatures on peri‐implant and periodontal tissues. Four cynomolgus monkeys received 2 ITI (Type F) dental implants in edentulous areas of each side of the mandible. Following a healing period of 2 months with regular prophylaxis procedures twice per week, plaque was allowed to accumulate. After 1 month (day 0), silk ligatures were placed around 1 of the 2 implants. The third molars served as controls and were ligated as well. Clinical examinations comprising Plaque Index (PlI), Gingival Index (GI), probing depth (PD) and loss of attachment (LA) were performed at the test and control sites before and 1, 2, 3, 4, 5, 6, 7 and 8 months after ligature placement. Sixty and 30 days prior to ligation and at 2, 5, 6 and 8 months following ligation, standardized radiographs were obtained and subtracted from a baseline radiograph obtained at the time of ligation. PI1 and GI scores increased significantly following the elimination of prophylaxis procedures. After ligation, these indices continued to increase and reached values significantly higher in ligated implant (LI) and ligated teeth (LT) sites than in non‐ligated teeth (NLI) sites. PD also increased after plaque accumulation and ligature placement. Significantly greater PD values were noted in LI and LT sites than in NLI sites. Also, loss of attachment was observed already after 1 month following ligation in LI and LT sites and reached a value of 3.8 mm after 8 months, whereas only 1.0 mm of attachment was lost after 7 months of plaque accumulation in NLI. Similar results were noted in the analysis of the subtraction images. LI and LT sites lost significantly more bone density and more alveolar bone height, but the NLI sites yielded relatively stable conditions throughout the study. It is concluded that the development of a peri‐implant infection progressed at a similar rate as the development of a periodontitis lesion and that ligature placement significantly promoted the development of these lesions when compared with the development of an inflammatory response in the peri‐implant tissues as a result of bacterial plaque accumulation.