Aim To identify predictors of compliance during non-surgical and supportive periodontal therapy (SPT). Materials and Methods In this retrospective study, demographic, dental, medical data of 427 new patients in a private practice were collected. Data were analysed in statistical models with non-surgical therapy and SPT compliance used as dependent variables. Results Of the 427 patients, 17.3% never agreed to initial therapy, 10.7% never completed therapy and 20.8% completed treatment, but never entered SPT. Of the 218 SPT patients, 56% became non-attenders after a period of 20 months, 33% were erratic attenders and 10.5% were regular attenders until the end of the observation period (5.5–6.5 years). Patients became erratic attenders after a mean period of regular attendance of 18.1 ± 16.2 months, whereas 49.6% of the patients, who abandoned SPT, were regular attenders until the time they stopped. In a univariate correlation model, periodontal disease severity emerged as a significant predictor of the completion of non-surgical periodontal therapy (p = 0.01). In a multivariate linear regression model, smoking was negatively associated with SPT compliance (p = 0.047). Conclusions A low compliance of the population was observed. Smoking and periodontal disease severity represented significant, but modest modifiers of a patient compliance with SPT and initial therapy respectively.