Various methods for evaluating tooth mobility have been developed throughout the years, but their acceptance has been limited because of the subjectivity associated with their use. In recent years, the Periotest has been studied and used to evaluate the mobility of natural teeth and is claimed to be potentially reliable in assessing the stability of the implant-bone interface. Few clinical implant studies have used natural teeth as controls to monitor changes in mobility associated with dental implants. The Dental Implant Clinical Research Group initiated a long-term clinical study in 1991 to assess the influence of design, application, and site of placement on clinical success and crestal bone height. As part of the study, Periotest values (PTVs) were recorded for 2623 of the 2998 implants placed and uncovered. For the statistical analysis, 2623 implants were tested at second-stage surgery, with the number of implants tested varying at each follow-up visit. Data were collected from investigators at 32 study centers for periods ranging up to 60 months. A total of 975 natural teeth from 409 partially edentulous study subjects served as controls. PTVs on natural teeth and implants were combined, and the overall average Periotest values (OA-PTVs) were compared with values for individual subjects. The effect of implant and natural tooth locations on mobility were evaluated and compared with each other. The combined OA-PTV for all natural teeth was +1.8 and the OA-PTV for all implants was −3.4 PTVs. Compared with those in the maxillae, mandibular teeth and implants were found to be more stable. Implants were found to be significantly more stable as compared with natural teeth. The recorded variations in PTVs for natural teeth and implants over the entire evaluation period were not found to be significantly different. This study developed the following conclusions: (1) implants were found to be significantly less mobile as compared with natural teeth for individual subjects; (2) the PTVs for natural teeth and implants did not exhibit significant variation over the evaluation period; (3) the Periotest can provide reproducible assessment of stability in a long-term clinical study; and (4) changes in PTVs may be helpful in evaluating improvement or degradation of the implant-bone complex.