The aim of periodontal treatment is to make and keep the periodontal tissues healthy. A proper diagnosis including the assessment of the different risk factors is essential before successful treatment can be carried out. Oral hygiene instruction together with supra- and subgingival cleaning is a basic part of the treatment. Antibiotic treatment and smoking cessation programs also contribute to a good clinical result. Until the seventies, when almost nothing was known about riskfactors and antibiotic therapy, no study was performed to assess the long-term effect of periodontal surgery being part of the treatment. Therefore a study was performed to assess clinically, radiographically and histologically the effect of periodontal surgery and oral hygiene on artificial periodontal defects. The results were reported in the thesis ‘Surgery and oral hygiene on artificial periodontal defects in beagle dogs’. This study established that periodontal surgery can contribute to a healthy periodontium. A good oral hygiene is a prerequisite. These results have been confirmed by a longterm study in private practice. Pocketdepths of 5 mm and more after initial treatment is a good criterion in the decision making for periodontal surgery.