A series of 908 cases of colonic carcinoma has been analysed to elucidate reasons for the poor prognosis in obstructing colonic cancer. Complete obstruction was present in 148 cases (16.3 per cent), 280 cases (30.8 per cent) had partial obstruction and 480 (52.8 per cent) presented without obstruction. There were fewer Dukes' A tumours in those with complete obstruction (P less than 0.005) and greater numbers of advanced tumours (P less than 0.0005) compared with those without obstruction. This is reflected in a lower curative resection rate of 50.7 per cent in those with obstruction compared with 70.6 per cent in those without obstruction (P less than 0.001). However, after curative resection there was no significant difference in the distribution of tumour stage. Patients with complete obstruction showed a higher incidence of recurrence (P less than 0.01) after curative resection, consequent to an increased incidence of local recurrence (P less than 0.02). Five-year cancer-specific survival for the total series was decreased from 59.1 per cent in patients without obstruction to 31.8 per cent in those with complete obstruction (P less than 0.001). After curative resection there was also a significant reduction in survival (P less than 0.001). It is concluded that completely obstructing colonic cancers are more aggressive than other colonic cancers.