The demonstration of pericryptal fibroblasts in background mucosa and dysplasia in ulcerative colitis was investigated by immunohistochemistry using monoclonal antibody for alpha-smooth muscle actin. Pericryptal fibroblasts were reduced in 18 (26%) of the 68 sections of non-dysplastic mucosa. The reduction was significantly correlated with goblet cell depletion and villous change. Pericryptal fibroblasts were more frequently reduced (50%) in dysplastic mucosa, the reduction being greater in villous than in non-villous dysplasia. Pericryptal fibroblast development was not related to grade of dysplasia (low or high-grade), distance from carcinoma (adjacent to or distant from carcinoma) or growth pattern (polypoid or non-polypoid). These findings suggest that: 1 reduction of pericryptal fibroblasts in background mucosa may relate to the development of dysplasia in ulcerative colitis, 2 reduction of pericryptal fibroblasts in villous regeneration, analogous to that in dysplasia, reinforces the hypothesis that villous change may be a marker for risk of development of carcinoma in ulcerative colitis.