One hundred and nine samples comprising carcinomas, adenomas, dysplastic, inflamed and normal mucosa from patients with sporadic colon cancer and ulcerative colitis (UC) were analysed for c-Ki-ras mutations. DNA was extracted from archival paraffin-embedded material, amplified using the polymerase chain reaction (PCR) and the PCR products analysed using restriction enzyme digestion. Forty-two per cent (14/33) of the sporadic carcinoma controls contained Ki-ras codon 12 mutations in contrast to 24% (8/33) of ulcerative colitis carcinomas. A significantly higher c-Ki-ras mutation rate was observed in rectal carcinomas (72%) in comparison to colonic carcinomas (28%) in control patients (P less than 0.04), while the opposite was observed in UC patients. The difference between the incidence of c-Ki-ras mutations in rectal carcinomas in UC (9%) and in sporadic rectal carcinomas (72%) was also significant (P less than 0.01). This lower prevalence rate and different site distribution of c-Ki-ras mutations in UC carcinomas compared to sporadic carcinomas suggests that specific genetic differences may underlie the causation of carcinomas arising in these situations.