BACKGROUND, AIMS The aim of the present investigation was to determine the success rates of treatment of 36 patients with moderate to advanced periodontal disease as related to different clinical and radiographic criteria. METHOD The treatment comprised oral hygiene education, subgingival scaling and root planing, and when judged indicated, periodontal surgery. An "evaluation criteria staircase" comprising 5 levels was introduced to be utilized for evaluation of the treatment results in 4 proximal sites (1 in each quadrant) which were followed for 3 years. The highest level of the staircase (level 1) is the most challenging and comprises the traditonal criteria for "perfect" periodontal health, while the lower levels are gradually less demanding. RESULTS At the follow-up examination 3 years after active treatment, 52.1% of the experimental sites fulfilled the criteria for successful treatment according to level 1. On the lowest level (level 5) which only required no further loss of alveolar bone for treatment to be considered "successful", the number of such treated sites reached 95.1%. The present data showed that even if level 5 was accepted as the evaluation criterion for "success", resulting in very few failed sites (4.9%), these sites may demand considerable extra clinical time and costs for retreatment because of their distribution among many individuals (13.2%). This clinical time increases substantially if higher levels of success are desired. From a cost-benefit point of view, it is therefore of utmost importance that not only patient compliance but also disease resistance and the value of the affected tooth for the dentition are taken into consideration when the indications for retreatment are weighed. This is especially important since many sites were found to be non-progressive for a long time, even though they did not exhibit perfect periodontal health. CONCLUSIONS It is suggested that the "evaluation criteria staircase" presented in this paper might be a helpful clinical instrument for decision-making in individually designed and site-related retreatments of patients with periodontal disease.