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Our objectives were to compare the stability of treatment and periodontal health with fixed vs removable orthodontic retainers over a 4-year period.A 4-year follow-up of participants randomly assigned to either mandibular fixed retainers from canine to canine or removable vacuum-formed retainers was undertaken. Irregularity of the mandibular anterior segment, mandibular intercanine and intermolar widths, arch length, and extraction space opening were recorded. Gingival inflammation, calculus and plaque levels, clinical attachment level, and bleeding on probing were assessed. The outcome assessor was blinded when possible.Forty-two participants were included in the analysis, 21 per group. Some relapse occurred in both treatment groups at the 4-year follow-up; however, after adjusting for confounders, the median between-groups difference was 1.64 mm higher in participants wearing vacuum-formed retainers (P = 0.02; 95% confidence interval [CI], 0.30, 2.98 mm). No statistical difference was found between the treatment groups in terms of intercanine (P = 0.52; 95% CI, −1.07, 0.55) and intermolar (P = 0.55; 95% CI, −1.72, 0.93) widths, arch length (P = 0.99; 95% CI, −1.15, 1.14), and extraction space opening (P = 0.84; 95% CI, −1.54, 1.86). There was also no statistical difference in relation to periodontal outcomes between the treatment groups, with significant gingival inflammation and plaque levels common findings.This prolonged study is the first to suggest that fixed retention offers the potential benefit of improved preservation of alignment of the mandibular labial segment in the long term. However, both types of retainers were associated with gingival inflammation and elevated plaque scores.Prolonged retention with fixed retainers was more effective than with vacuum-formed retainers.Periodontal outcomes were similar in the treatment groups 4 years after treatment.