Recurrent aphthous stomatitis (RAS) and oral lichen planus (OLP), two of the most common stress-related ulcerations of the oral mucosa, have distinct etiologies, courses, and histopathologic features. In light of increased understanding of the psychobiology of stress, the authors propose that RAS and OLP fundamentally differ from the perspective of the psychobiologic response to stress. This article presents possible clinical implications.