According to theory, health beliefs are related to health behaviors. We investigated whether individuals who hold favorable oral-health-related beliefs over time have better adult oral health than those who do not. Beliefs about the efficacy of water fluoridation, keeping the mouth clean, avoiding sweet foods, visiting the dentist, using dental floss, and using fluoridated toothpaste were assessed in a birth cohort at ages 15, 18, and 26 years. At each age, the majority of participants endorsed the importance of each practice. However, there was also evidence of instability across time. Individuals who held stable favorable dental beliefs from adolescence through adulthood had fewer teeth missing due to caries, less periodontal disease, better oral hygiene, better self-rated oral health, and more restorations. Dental beliefs can change between adolescence and young adulthood, and these changes are related to oral health. In particular, unfavorable dental health beliefs are related to poorer oral health.