This study is based on all patients with ulcerative colitis from a defined catchment area in Northern Sweden in a still ongoing colonoscopy surveillance programme, which started in 1977. From this material we selected tissue from eight groups of patients consisting of normal control biopsies (5), inactive colitis (10), active colitis (10), findings of low-grade dysplasia (10), high-grade dysplasia (6), aneuploidy (without dysplasia and with subsequent dysplasia) (10), and ulcerative colitis-associated cancers (5). The samples were evaluated according to immunohistochemical expression of CK7 and CK20. Colonic mucosa from normal controls and inactive colitis was found to be completely negative for CK7. In 9 out of 10 patients with active colitis, CK7 was sparsely expressed in a patchy manner and connected with active epithelial inflammatory areas. 7 out of 10 patients with low-grade dysplasia and 3 out of 6 with high-grade dysplasia were positive for CK7. Samples with aneuploidy without dysplasia were completely negative, while 2 out of 6 showing subsequent dysplasia were positive. Of the five cancers, two were positive for CK7. CK20 was expressed in nearly all samples but relatively more in the lower part of the crypts in neoplasia-associated lesions. Our results indicate a possible relationship between expression of CK7 and CK20 and neoplastic development of colorectal mucosa in patients with ulcerative colitis. Further studies are needed to elucidate whether these findings have clinical significance.