Mucosal biopsies from patients with Crohn's disease and with ulcerative colitis were studied by scanning electron microscopy. Important abnormalities of the mucosal surface were found in both diseases. For Crohn's disease, the characteristic abnormality was loss of the regularity of the polygonal units, but with preservation of the mucosal integrity and of the normal mucosal design. For ulcerative colitis, the abnormalities were disorganization of the cells, signs of sloughing, and superficial erosions. Patients with Crohn's disease always had a significantly increased number of muciparous cells, while those with ulcerative colitis had obvious signs of decreased mucus production. The lesions of ulcerative colitis could be seen under the scanning electron microscope in mucosal areas that appeared normal endoscopically. We feel therefore that scanning electron microscopy of biopsy specimens from patients with inflammatory bowel diseases can be of great help in differential diagnosis.