Small (less than 1 mm), superficial erosions ('microerosions') have been observed stereo-microscopically in surface-stained rectal biopsies in Crohn's disease (CD). Biopsy specimens from 97 patients with CD, 225 with ulcerative colitis (UC), and a control material of 161 patients were investigated to define the occurrence and significance of the microerosions. In CD they were found in 13% of specimens, in UC in 5%, and in the control material not at all. In CD they were observed only in patients with colonic involvement and were most frequent (44%) in patients with this site of affection exclusively. Granulomas were identified in 62% of the biopsies with microerosions and by examination of two consecutive biopsies from each of these patients, in 85% indicating a positive correlation. In patients with microerosions and a primary diagnosis of UC, granulomas were found in 38% and by examination of two biopsies in 54%. Patients with granulomas and a few other patients were reclassified as CD, but there still remained some patients with microerosions, who most probably had UC. In conclusion, microerosions are observed mainly in CD with colonic involvement. There is a high incidence of granulomas in biopsy specimens with microerosions. The demonstration of microerosions in a specimen is suggestive of, but probably not diagnostic of, CD.