Rectal mucosal dysplasia in Crohn's disease. Serial sections of 812 rectal biopsy specimens from 356 Crohn's disease patients were analysed for mucosal epithelial dysplasia. Dysplasia was found in 18 patients (5%), with four showing dysplasia on repeat biopsy specimen. In these 22 biopsy specimens the dysplasia was mild in 13, moderate in nine, and severe in none. Subsequently, three patients (17%) developed neoplasms including carcinoma in two and an adenomatous polyp in one. In colectomy specimens which showed dysplasia, significantly more dysplastic changes were found in seven patients who underwent colonic resection than in 10 others who underwent operation but had no prior dysplasia (p less than 0.001). Thirteen patients still have their rectum in situ and remain at risk of developing colonic cancer. Four carcinomas developed in patients with Crohn's disease who did not have dysplasia on rectal biopsy specimen. 