In this study, 52 miniature schnauzers (age 1.3–6.9 years) were anesthetized and assessed for gingivitis and periodontitis every 6 weeks for up to 60 weeks. Before the study, all dogs received an oral care regimen of tooth brushing every second day from approximately 1 year of age. Tooth brushing was stopped 1 week (n = 42 dogs) or 18 weeks (n = 10 dogs) before the first dental assessment. At the beginning of the investigation, some level of gingivitis was present in all of the 2155 teeth examined; 23 teeth showed periodontitis. Over the 60-week period, the number of teeth within a dog that developed periodontitis ranged from 0 to 21. Thirty-five dogs developed periodontitis in at least 12 teeth, the most common of which were incisors, followed by fourth premolars and first molars. Only 1 dog did not develop periodontitis in any teeth. Progression was more rapid in older dogs. This study highlighted the importance of an oral care routine and semiannual dental examinations starting at a young age in miniature schnauzers and other breeds with similar predicted rates of periodontal disease.