To assess the association of mucosal dysplasia and colonic cancer in patients with ulcerative colitis and to avoid bias in biopsy interpretation that may have affected results of previous studies, the authors examined coded histology slides from colectomy specimens of 22 patients who had ulcerative colitis and colonic cancer and 22 patients who had ulcerative colitis but no colonic cancer. As expected, it was found that dysplasia occurred contiguous to each cancer. However, at a distance from the cancer (i.e., in histology blocks not containing cancer), some dysplasia was found (low or high grade) in 16/22 cases (73 percent), and high grade dysplasia in 11/22 cases (50 percent). These results suggest that there may be somewhat weaker association than previously reported between colonic cancer and dysplasia at a distance from the colonic cancer. Further, these results suggest that, in studies of dysplasia, it is important to avoid bias in biopsy interpretation and to describe sampling methods.