Fifty sporadic colorectal carcinomas diagnosed in 1991 were anal, scd for microsatellite instability at four loci. Five of 50 (10 per cent) tumours showed replication errors (RERs) at two or more loci and were classed as RER-positive (RER+). A further five showed RERs at one locus only. A significant association (Fisher exact test) was found between RER+ rumours and location in the proximal colon, exophytic shape, size >5 cm, histological margin, lymphoid reaction, and near-diploid DNA content, There nas no significant difference for age, ses, grade, mucin production, p53 protein overexpression or Duke's stage. There was no significant difference in survival as measured over a 60-month follow-up period. The findings, though Limited by the small case numbers involved, show an association between RER positivity. in sporadic colorectal tumours and certain clinico-pathological features. They do not suggest a better clinical outcome for sporadic RER+ rumours. Copyright (C) 1999 John Wiley & Sons, Ltd.