The biopsy diagnosis of prolapsing rectal mucosa syndrome can be difficult. We present two newly described features--'diamond-shaped' crypts and mucosal elastin--which appear to be helpful in histological diagnosis. Of 32 biopsies of prolapsing rectal mucosa syndrome, all showed diamond-shaped crypts or mucosal elastin, and 28 contained both. Control biopsies comprised cases of normal or irritable bowel syndrome (46), irradiation colitis and ischaemic colitis (16), inflammatory bowel disease (26), and adenomas (30). Mucosal elastin and 'diamond-shaped' crypts with distinctive scalloped edges, which were never seen in prolapse, were observed in half the cases of irradiation and ischaemic colitis. Diamond-shaped crypts were seen in one case of inflammatory bowel disease. Diamond-shaped crypts and elastin were seen in the base of adenomas large enough to cause localized prolapse, and in four biopsies from patients with irritable bowel syndrome, all of whom had given a history of straining at stool.