Objective The aim of this retrospective analysis was to evaluate, in adult patients treated for periodontitis, the periodontal stability during supportive periodontal therapy (SPT). Methods Data were collected and analyzed retrospectively for periodontitis patients aged ≥36 years who underwent active periodontal therapy (APT) and were following an SPT programme. The stability of the APT success, defined as a probing pocket depth (PPD) of ≤5 mm, was the main outcome parameter. Analyses were performed in which PPD, tooth loss (TL), bleeding on probing (BOP), periodontal epithelium surface area (PESA), and the effects of age, gender, smoking status, and the number of years in SPT were evaluated. The annual TL and BOP of <10% in addition to a PPD of ≤5 mm were considered to be secondary outcome variables. Results In total, 993 patients were included, in 36% of whom a PPD ≤5 mm was found at the evaluation of APT. If the outcome was defined as a BOP of <10% in addition to a PPD of ≤5 mm, this was present in only 16% of the patients. During SPT, a small overall increase in clinical parameters for the total population and an annual average TL of 0.15 per patient was observed. Patients of male gender and smokers negatively affected the success of SPT. Conclusion The periodontal clinical status remained ‘fairly’ stable during SPT in chronic periodontitis patients aged ≥36 years. Smoking negatively affects the outcome of APT and periodontal stability during SPT.