About 1% of dividing epithelial cells in esophageal, gastric and duodenal mucosas with chronic inflammation have been found to be atypical. In the present work, the characteristics of the mitotic figures in the colorectal mucosa with ulcerative colitis (UC)--a disease characterized by chronic mucosal inflammation--were investigated. Feulgen-stained histologic sections (having > 30 mitoses in 3 large tissue sections) from 59 colectomy specimens with long standing total UC were scrutinized at high power microscopy (1000 x). A total of 2,104 mitoses were recorded in the 59 specimens with UC: 591 (28.1%) were atypical. Atypical mitoses were present in 19.2% (SE 4.7) of the 26 specimens having chronic active inflammation, in 16.9% (SE 5.6) of the 9 with UC in remission, in 34.6% (SE 7.7) in mucosas with dysplasia (n = 8) and in 49.1% (SE 10.2) in carcinoma (n = 16). Atypical mitoses were also found in 17.0% (SE 5.4) in the non-dysplastic mucosa from colitic patients having dysplasia or carcinoma elsewhere as well as in 0.96% (SE 0.3) in 14 "noncolitic" controls with chronic inflammation. The percentage of atypical mitoses was influenced-except in areas with dysplasia or carcinoma-by the duration of the disease (< 15 or > 16 years), but not by the age or the gender of the patients. Since the frequency of atypical mitoses in "noncolitic" controls with chronic inflammation was low, it is evident that factors other than chronic inflammation may be involved in the induction of atypical mitoses in colitics. Acquired nuclear aberrations detected during the mitotic phase, but not at interphase do occur in colorectal epithelial cells from long-standing UC patients, not only in areas with dysplasia or carcinoma but also in areas without those neoplastic changes.