BACKGROUND AND OBJECTIVE: Degree III furcation involvement (FI) represents a risk of molar tooth loss. A limited number of studies have assessed the survival of molars with degree III FI treated with tunnelling procedures. AIMS: The aim of the present study was to assess periodontal disease progression and tooth loss in a cohort of patients with degree III FI treated with tunnelling by two periodontists in a private practice setting in the UK and in a hospital setting in Sweden. MATERIALS AND METHODS: A retrospective study was conducted on 102 consecutive surgically-created tunnelled molars in 62 periodontitis patients and followed up at least 5 years later (average 7 years and 9 months follow-up). RESULTS: Overall tooth loss for tunnelled molars was 29.4%. Multivariate analysis revealed statistically significant associations with tooth loss for 'irregular supportive periodontal therapy (SPT)' (p=0.039) and age (p=0.037). Tooth loss occurred only in the Swedish sample, not undergoing regular SPT. CONCLUSION: A high rate of tooth loss was observed following tunnelling surgery, mainly in patients not undergoing regular supportive therapy. Clinical studies should be carried out to compare tunnelling with other treatment options for advanced furcation involvement in patients on SPT. This article is protected by copyright. All rights reserved.