Short chain (C2-C6) fatty acids are produced in the colon through bacterial fermentation of mainly dietary fibre. Butyrate (C4) possesses antineoplastic effects on human colon carcinoma cells, and epidemiological studies indicate that high fibre diets may reduce the incidence of colorectal cancer. The role of dietary fibre during colorectal carcinogenesis might therefore be related to its fermentation to butyrate. Faecal concentrations of total short chain fatty acids and concentrations and ratios of the individual C2-C6 fatty acids did not differ between 16 healthy controls, 17 patients with colonic adenomas, and 17 patients with colonic cancer. Comparison of the molar production velocities (mmol/l.hour) of total and individual short chain fatty acids from glucose, ispagula, wheat bran, and albumin in six and 24 hour faecal incubations showed no differences. The ratio of butyrate production to total short chain fatty acid production from fibre, however, was reduced in patients with colonic cancer and adenomas compared with healthy controls (ispagula, six hours: 6.4, 7.6, and 11.5% respectively, p = 0.005 and 24 hour: 9.1, 9.9, and 15.4%, p = 0.002; wheat bran, six hours: 9.9, 10.2, and 14.7% respectively, p = 0.06 and 24 hours: 15.1, 16.8, and 21.0%, p = 0.01). It may be that the low ratios of colonic butyrate formation combined with low fibre diets increase the risk of colonic neoplasia.