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 (greater than 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 less than 0.01), they were more likely to be Dukes' stage 'D' (58 versus 31 per cent: P less than 0.01) and local fixity was commoner (70 versus 37 per cent: P less than 0.001). Consequently the overall resection rate was reduced from 90 to 73 per cent (P less than 0.01), the curative resection rate from 69 to 42 per cent (P less than 0.01) and the 5-year survival rate from 37 to 18 per cent (P less than 0.05). Colorectal carcinomas of high mucin content require wide excision, tend to recur locally and carry a poor prognosis.