The present clinical study was initiated in 1969 to test the hypothesis that microbial plaque is the cause of periodontal disease and that hence even advanced cases of periodontitis can be cured in patients willing to exercise optimal plaque control. The material consisted of 75 patients with severe destruction of the periodontal tissues. The patients were selected for the study because of their ability to maintain plaque-free dentition. Following an initial examination, a preliminary treatment plan was presented to the patients. This plan included detailed instructions in plaque control measures, scaling, root planning and emergency dental care, including endodontic therapy and extractions. Three to six months after the termination of the so-called presurgical treatment, surgical elimination of pathologically deepened pockets was performed. The patients were recalled every 3 to 6 months after the end of the treatment. At these reexaminations the following parameters were assessed: Plaque Index, Gingival Index, Pocket Depths, Marginal alveolar bone topography and height. The results demonstrate that it is possible to treat periodontal disease successfully, even in advanced stages, in patients willing to maintain plaque-free dentition.