Few data are available regarding the long-term impact of supportive implant therapy (SIT) on peri-implant diseases. We present long-term results on the possible effects of SIT on the peri-implant tissues. In a private practice, 50 consecutive patients with 101 implants had attended an SIT programme for >6 years (SIT group). A second consecutive group comprised patients without SIT compliance (C group). We assessed pocket probing depth (PPD), bleeding on probing (BOP), bone loss (BL) and implant hygiene (QHI). Correlations between SIT compliance and the prevalence of peri-implant diseases were evaluated statistically. The inclusion criteria were met by 48 patients with 98 implants (SIT group) and 43 patients with 121 implants (C group). SIT group: Mucositis rate 30%; mean PPD 3.76 ± 0.86 mm; bone loss 1.02 ± 0.85 mm, peri-implantitis rate 4%. C group: Mucositis rate 68%; mean PPD 4.07 ± 1.18 mm; bone loss 1.53 ± 1.46 mm; and peri-implantitis rate 17%. Significant associations were found between SIT compliance and reduced peri-implant mucositis (p = .019), reduced highest PPD (p = .038) and peri-implantitis rates (p = .027). Patients under SIT were associated with lower incidence of peri-implant diseases. Patients without regular maintenance might exhibit a 4.25-fold increased risk for peri-implantitis. Therefore, SIT programmes should be considered essential. © 2019 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.