To evaluate the prevalence of peri-implantitis (PI) and peri-implant mucositis (PM) in a long-term follow-up with comparison among different PI and PM definitions, and to report on the incidence of PI. In a retrospective clinical study five different PI and PM definitions were applied onto a population with 274 implants 17 to 23 years postimplant placement. Recommendations by the Eighth European Workshop on Periodontology (EWOP) were used as base reference. Clinical and radiological measurements were considered. Risk factors were evaluated in a regression analysis. After an average observation period of 18.9 years, 40.1% of the implants were diagnosed with PM and 15.0% with PI (Eighth EWOP). PI incidence reached 7.9% on implant level and 13.2% on patient level. Implants diagnosed with PI and progressive bone loss displayed exceptionally vertical bone defect configuration (BDC). Diabetes mellitus, smoking, regular maintenance, or a former periodontal infection did not show significant influence on the prevalence of peri-implant diseases. Patients with bruxism displayed significantly less PM and PI. Vertical BDC seems to correspond with active PI, wherefore we estimate such a defining factor of importance. Diagnosis of PM and evaluation of probing pocket depths might be only of descriptive interest as they could lead to false-positive results. © 2019 The Authors. Clinical Implant Dentistry and Related Research Published by Wiley Periodicals, Inc.