Insufficient wear of rodent incisors often results in malocclusion and rapid tooth elongation. This condition may go undetected for a prolonged time and have a negative effect on affected animals' welfare. Dental overgrowth can lead to undernutrition due to chronic difficulty in feeding and may cause trauma to the surrounding tissues, potentially ultimately resulting in death. Here we describe the increased incidence of malocclusion observed during a longitudinal study of the normal growth and aging of Wistar rats. Histologic examination of the temporomandibular joint of affected animals did not reveal any inflammatory or degenerative changes. Because no environmental factor could be implicated in the condition, we considered that aging or genetic factors were responsible for its appearance. We conclude that special attention should be given to the potential appearance of malocclusion during long-term rodent studies, because its incidence may adversely affect the animals' health and general wellbeing.