This study prospectively evaluated the effect of keratinized mucosa (KM) on the healing of the peri- implant tissues after resective surgical therapy for peri-implantitis. It addressed the question of whether the absence of KM has a negative effect on peri-implant tissue stability after treatment. Patients referred to a private practice for peri-implantitis treatment were surgically treated with resective flap surgery and implantoplasty. They were followed for 3 years and measurements at implant level included presence of plaque, bleeding on probing, probing pocket depth, radiographically visible bone loss and the presence and width of KM before therapy, 3 month post-operatively, 1 year, 2 years, and 3 years after the surgical intervention. All clinical parameters improved and marginal bone levels remained stable 3 years after therapy. The width of the KM decreased significantly after the therapy. Soft tissue recession was frequently observed. The absence of KM does not seem to have a negative effect on healing after therapy for the studied period of 3 years. The authors conclude that resective surgery combined with implantoplasty seems to be a reliable method for arresting the progression of peri-implantitis.