Submucosal edema and lymphectasia have traditionally been considered the earliest recognizable alterations in Crohn's disease. However, a characteristic pattern of ulceration grossly resembling the oral lesions of aphthous stomatitis is believed by others to be the earliest macroscopic lesion. We have studied 50 consecutively accessioned surgical specimens with Crohn's disease in an effort to define more thoroughly the frequency, distribution, and morphology of these "aphthoid" ulcers. The scanning electron microscope (SEM) was used in the study because an understanding of the morphology and evolution of these lesions requires an appreciation of their three-dimensional configuration. Typical "aphthoid" ulcers were identified in 35 of the 50 specimens studied. Grossly the typical ulcerative lesion varies from barely visible up to 3 mm in diameter. They have a characteristic light-microscopic appearance consisting of focal ulceration usually overlying an aggregate of lymphoid tissue. The SEM was helpful in identifying the smallest of these lesions and was especially useful in defining a variety of villous abnormalities in the small bowel mucosa adjacent to the ulcers.