Objectives: To evaluate possible benefits of the adjunctive use of light-activated disinfection (LAD) in the treatment of peri-implantitis. Materials and methods: Eighty patients with at least one implant affected by peri-implantitis defined as at least 3 mm of bone loss from the baseline radiographs in the presence of signs of infection (pus exudation and/or soft tissue swelling and/or soft tissue redness) were nonsurgically or surgically treated. A total of 50% were randomly allocated to receive an additional treatment of LAD (FotoSan) according to a parallel group design at four different centres. Only one implant per patient was considered. Outcome measures recorded up to 4 months after treatment were implant failures, complications, probing pocket depth (PPD) changes and number of re-treatment sessions recorded by blinded assessors. Patients were followed for 4 months after treatment. Results: Five treated patients did not fit the original inclusion criteria, 4 because they were not affected by peri-implantitis (as defined in the present study) and one because the patient was treated with antibiotics, however they were included according to an intention-to-treat analysis concept. Nine patients of the LAD group were treated surgically versus 10 control patients. After 4 months, 1 patient dropped out from the LAD group and no implant failed. Fifteen implants had to be re-treated in the LAD group versus 16 implants in the control group, the difference was not significant. Three minor complications occurred: 2 in the LAD group and 1 in the control group. PPD significantly decreased in both groups, and at 4 months there was no significant differences between groups (difference -0.12 mm, 95% CI of difference -0.94 to 0.69, P = 0.77). There were significant differences between centres for number of re-treatments needed, PPD changes, plaque and marginal bleeding 4 months after treatment. The results did not change when removing the 5 patients who did not match the present inclusion criteria. Conclusions: The use of adjunctive LAD therapy with mechanical cleaning of implants affected by peri-implantitis did not improve any clinical outcomes up to 4 months after treatment, however longer follow-ups are needed to evaluate the possible effect of repeated LAD therapy over time. Keywords: infection, light activation disinfection, peri-implantitis