Erythrocyte stearic acid desaturation in patients with colorectal carcinoma. The erythrocyte stearic:oleic acid ratio (saturation index) was investigated as a means of differentiating between control subjects (n = 146) and patients with benign (n = 48) and malignant (n = 117) colorectal disease and patients undergoing postoperative follow-up after curative resection (n = 49). Erythrocyte fatty acid profiles were determined by gas liquid chromatography. Neither age, sex, Dukes' stage, nor degree of differentiation of the tumors had a significant effect on the erythrocyte saturation index. The erythrocyte saturation index was lower in patients with primary and recurrent colorectal cancer compared with control subjects and patients with inflammatory bowel disease or benign colonic polyps (P less than 0.0001). The erythrocyte saturation index was not found to be useful in the postoperative follow-up of these patients. Using both saturation index and age as a means of differentiating between patients with primary colorectal cancer and control subjects gave a sensitivity of 67 percent and a specificity of 81 percent. 