Patients with extensive ulcerative colitis are entered into surveillance programs that aim to detect premalignant changes. Biopsy specimens have been collected in the St Mark's Hospital (London) surveillance program over a 22-year-period. Specimens from patients reported as having dysplasia were reexamined. A total of 207 biopsy specimens from 86 patients were graded by five experienced pathologists according to the severity of the dysplasia. The overall agreement between the pathologists grading the specimens was poor; each pair agreed on between 42% and 65% of the slides. The best agreement was for slides that were said to show no dysplasia. Comparison with clinical outcome indicated that the pathologists most likely to diagnose dysplasia in patients with carcinoma were also likely to diagnose dysplasia in patients who did not go on to develop carcinoma. Calculating an average grade of dysplasia did not significantly improve diagnostic accuracy. Despite the findings of this interobserver study, dysplasia has been a successful marker in clinical practice. Pathologists should ensure that they have access to previous slides from the same patient and adequate clinical information before reporting biopsies as positive for dysplasia. An additional biopsy should usually be undertaken before surgery is considered.