Erythrocyte fatty acids were measured in 32 patients with colorectal cancer (four with recurrent disease), four patients with large colorectal adenomas, and 42 hospitalised patients without cancer. No significant differences were found with respect to any of the fatty acids between these groups. In particular there was no difference between the erythrocyte stearic acid: oleic acid ratios (mean +/- SD) between the cancer cases (1.07 +/- 0.31) and the control cases (1.09 +/- 0.31, 2-tailed Mann-Whitney U test). No difference in this ratio was observed with respect to age or sex and in the case of colorectal cancer, to the Dukes' stage, degree of tumour differentiation, or recurrence. A significant correlation was found between stearic acid concentrations in plasma and erythrocytes from fasting individuals (r = 0.477, n = 24, P less than 0.05). This study provides further evidence that the erythrocyte stearic acid: oleic acid ratio is of no value for diagnosing primary or recurrent colorectal cancer.