Objective: Many diseases are characterised by their seasonal appearance due to circannual changes in immune defence and a lifestyle that changes over the seasons. However, there is a lack of studies regarding the influence of seasonality on periodontitis. Therefore, the aim of this non-interventional, retrospective, observational study was to investigate the influence of seasonality on periodontitis. Materials and methods: Periodontal parameters of 5,908 patients of a practice-based research network (PBRN) were analysed. Probing pocket depth (PPD), Bleeding on Probing (BOP) and tooth mobility were assessed for seasonal fluctuations. Results: Within the PBRN patient data, seasonality was significantly associated with PPD of the individual months in patients with periodontitis. Pairwise comparison between the months showed significantly higher PPD in July compared to several months. PPD appeared significantly lower in December compared to June and November. Regarding season, the proportion of deep PPDs (6 mm) was significantly higher in summer compared to autumn (p = 0.024). Concerning BOP, a significant difference between winter and autumn could be observed (p = 0.016). No seasonal influence was found for tooth mobility. Conclusion: This practice-network based study indicated a tendency for seasonal variations in periodontal parameters among periodontal patients. However, the differences did not achieve clinical relevance. Clinical relevance: Regarding the seasonality of several diseases there might also be an influence of seasons on the periodontium, which would have a potential influence on periodontal studies and daily periodontal examination.