The present investigation describes the effect of periodontal therapy in a group of patients who, following active treatment, were monitored over a 5-year period. One aim of the study was to analyze the rôle played by the patients' self-performed plaque control in preventing recurrent periodontitis. In addition, probing depth and attachment level alterations were studied separately for sites with initial probing depths of greater than or equal to 4 mm which were treated initially by either surgical or non-surgical procedures. Following active treatment (surgical/non-surgical), the patients were maintained on a plaque control regimen for 6 months, which included professional tooth cleaning once every 2 weeks. During the subsequent 18 months, the interval between the recall appointments was extended to 12 weeks and included prophylaxis as well as oral hygiene instruction. Following the 24-month examination, the interval between the recall appointments was further extended, now to 4-6 months. In addition, the maintenance program was restricted to oral hygiene instruction and professional, supragingival tooth cleaning, but further subgingival instrumentation was avoided. Clinical examinations including assessments of the oral hygiene, the gingival conditions, the probing depths and the attachment levels were performed at Baseline and after 24 and 60 months after completion of active therapy. Assessments of plaque and gingivitis were repeated annually. The results of the examinations showed that the patients' standard of self-maintained oral hygiene had a decisive influence on the long-term effect of treatment. Patients who during the 5 years of monitoring consistently had a high frequency of plaque-free tooth surfaces showed little evidence of recurrent periodontal disease, while patients who had a low frequency of plaque-free tooth surfaces had a high frequency of sites showing additional loss of attachment. The present findings demonstrated that sites with an initial pocket depth exceeding 3 mm responded equally well to non-surgical and surgical treatments. This statement is based on probing depth and attachment level data from sites which were free of plaque at the 6-, 12-, 24-, 36-, 48-, and 60-month reexaminations. It is suggested that the critical determinant in periodontal therapy is not the technique (surgical or non-surgical) that is used for the elimination of the subgingival infection, but the quality of the debridement of the root surface.