Abstract The clinical and pathological features of 54 mucinous carcinomas of the large intestine were compared with those of 576 non-mucinous carcinomas. Tumours were only categorized as mucinous if they contained at least 60 per cent of mucin by volume. Those with a moderate mucin content (60–80 per cent) were indistinguishable in behaviour from ‘non-mucinous’ tumours. By contrast, those with a high mucin content (> 80 per cent) showed several differences from non-mucinous cancers: they had a more proximal distribution through the large intestine, they comprised a greater fraction of cancers in the under 50 age group (24 versus 7 per cent: P < 0·01), they were more likely to be Dukes' stage ‘D’ (58 versus 31 per cent: P < 0·01) and local fixity was commoner (70 versus 37 per cent: P < 0·001). Consequently the overall resection rate was reduced from 90 to 73 per cent (P < 0·01), the curative resection rate from 69 to 42 per cent (P < 0·01) and the 5-year survival rate from 37 to 18 per cent (P < 0·05). Colorectal carcinomas of high mucin content require wide excision, tend to recur locally and carry a poor prognosis.