The purpose of this study was to compare the clinical and microbiological (microscopic) parameters during the development of experimental gingivitis and experimental peri-implant mucositis. Twenty partially edentulous patients were treated for moderate to advanced periodontal disease. Following active periodontal therapy consisting of motivation, instruction in oral hygiene practices, scaling and root planing and periodontal surgery where indicated, IMZ oral implants were incorporated in posterior edentulous areas. After 3 months of healing, the prosthetic abutments were connected, and the patients were closely supervised for another 2 months of healing. At this time, baseline data were obtained. Re-examinations were scheduled at 3 and 6 months. Following this, the patients were asked to refrain from oral hygiene practices for 3 weeks. At all examinations including the end of the period of no oral hygiene, the following clinical parameters were obtained: Plaque Index, Gingival Index and Sulcus Bleeding Index, all modified by Mombelli et al. (1987), probing pocket depths and recession in mm. Furthermore, submucosal/subgingival plaque samples were obtained and analyzed using phase contrast microscopy. At the end of the 3-week period of plaque accumulation, optimal oral hygiene was reinstituted. There were no statistically significant differences between the mean values of all parameters at implant compared to tooth sites at any observation periods. The period of no oral hygiene demonstrated a similar cause-effect relationship between the accumulation of bacterial plaque and the development of peri-implant mucositis as established for the gingival units by the experimental gingivitis model.