Mucus secreted by colorectal cancer differs in three respects from that produced normally: an overall reduction, a loss of O-acetyl substituents in sialic acid, and an increase in neutral mucin. Similar changes have been reported in apparently normal mucosa bordering colorectal cancer. "Normal" left sided colorectal mucosa from 32 patients with rectal cancer was studied. Each case was matched by age and sex to a patient with diverticular disease and a patient with irritable bowel syndrome. Twenty five patients with right sided cancer were matched to patients with Crohn's disease. Sections were stained with mild periodic acid Schiff (mPAS) (selectively stains N-acetyl sialic acid lacking in O-acetyl group) and other closely related techniques. Reactions were graded negative, weak, and intense. An intense reaction was found in 9% of cases; there was no difference between the various matched groups. Phenylhydrazine interposition failed to block the mPAS effect, indicating that a positive result was due to a deficiency of sialic acid with O-acetyl substituents rather than neutral mucin. Different staining patterns in left and right colon were probably due to differing ratios of total sialic acid:fucose. These findings indicate a hitherto unsuspected colorectal goblet cell sialomucin heterogeneity within the general population, but no association with neoplastic disease is apparent.