A clinical trial was undertaken to test the hypothesis that periodontitis can be cured and that bone regeneration occurs in infrabony pockets in patients maintained on an optimal standard of oral hygiene. The material comprised 24 patients with advanced periodontal disease. After an initial examination, the patients were randomly distributed into one test group and one control group. All the patients were given instruction and practice in a proper oral hygiene technique, and then subjected to periodontal surgery using the modified Widman flap procedure. Following treatment, during a 2-year period the patients of the test group were recalled once every second week for professional tooth cleaning. The control patients were recalled once every 12 months for prophylaxis. The results showed that all osseous defects of the patients of the test group were refilled with bone. The control patients, on the other hand, could not maintain a high standard of oral hygiene, and exhibited a progressive deterioration of the periodontal tissues during the postsurgical observation time.