Purpose: To evaluate possible benefits of the adjunctive use of light-activated disinfection (LAD) in the treatment of peri-implantitis. Materials and methods: A total of 80 patients with at least one implant affected by peri-implantitis defined as at least 3 mm of bone loss on baseline radiographs in the presence of signs of infection (pus exudation and/or soft tissue swelling and/or soft tissue redness) were non-surgically or surgically treated for peri-implantitis and 50% of them were randomly allocated to receive an additional LAD treatment (FotoSan) according to a parallel group design at four different centres. Only one implant per patient was considered. Outcome measures were implant failures, recurrence of peri-implantitis, complications, peri-implant marginal bone level (RAD) changes, probing pocket depth (PPD) changes and number of re-treatment sessions recorded by blinded assessors. Patients were followed up for 1 year after treatment. Results: Five treated patients did not fit the original inclusion criteria: 4 because they were not affected by the present definition of peri-implantitis and 1 due to being 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 1 year, 3 patients dropped out, all from the LAD group. One implant treated with the LAD therapy failed versus none of the control group. Four complications occurred: 3 in 3 patients of the LAD group and 1 in the control group. Recurrence of peri-implantitis defined as 2 mm of peri-implant bone loss or more recorded on standardised periapical radiographs was observed in 6 patients, 3 from each group. In total, 29 implants were re-treated 1 to 4 times in the LAD group versus 33 implants 1 to 4 times in the control group; the difference was not statistically significant. Peri-implant marginal bone levels remained stable up to 1 year with no statistically significant differences between groups (0.13 mm favouring LAD therapy; 95% CI of difference -0.47 to 0.72; P = 0.68). PPD significantly reduced in both groups, and at 1 year there were no significant differences between groups (difference 0.19 mm favouring LAD therapy; 95% CI of difference -0.70 to 1.07; P = 0.68). There were significant differences between centres for the number of re-treatment sessions delivered, PPD changes, plaque and marginal bleeding 1 year after treatment, but not for implant failures, complications, RAD changes and recurrence of peri-implantitis. The results did not change when removing the 5 patients who did not match the original inclusion criteria. Conclusions: Adjunctive use of LAD therapy (FotoSan) with mechanical cleaning of implants affected by peri-implantitis did not improve any clinical outcomes when compared to mechanical cleaning alone up to 1 year after treatment. Keywords: infection, light-activated disinfection, peri-implantitis