The prognosis in colorectal cancer is related to the stage of the tumour. Thus, early detection of developing tumours will significantly improve the overall prognosis. This study assessed the histochemical changes in mucus from normal and tumour bearing mucosa to determine possible premalignant changes. Eighty patients with colorectal cancer and ten normal subjects were studied. Biopsies were taken from the tumour and from adjacent 'normal' mucosa and from both resection edges. Sections were stained with high iron diamine-alcian blue to distinguish sulphated from sialomucins. All normal mucosal biopsies showed a predominantly sulphated mucin pattern. By contrast, all 70 tumour mucosal biopsies showed a marked sialomucin staining. In transitional mucosa adjacent to the tumour, 30 patients showed marked increase in sialomucin (2+), 42 a moderate increase (+) and the remaining eight a normal pattern (-ve). Twenty-one (15%) patients had increased sialomucin at either surgical resection margins, all of whom had similar changes adjacent to the tumour, suggesting a wide field change in the mucus pattern. Since these changes in mucus are associated with malignant transformation then estimation of mucus pattern may give early prediction of malignant change in patients at risk of developing primary or recurrent carcinoma.