This multicentre study is the first prospective randomized trial to compare subtotal colectomy with segmental resection and primary anastomosis following intraoperative irrigation for the management of malignant left-sided colonic obstruction. Of the 91 eligible patients recruited by 12 centres, 47 were randomized to subtotal colectomy and 44 to on-table irrigation and segmental colectomy. Hospital mortality and complication rates did not differ significantly, but 4 months after operation increased bowel frequency (three or more bowel movements per day) was significantly more common in the subtotal colectomy group (14 of 35 versus four of 35, chi 2 = 6.06, 1 d.f., P = 0.01). More patients in the subtotal colectomy group reported that they had consulted their general practitioner with bowel problems than did those in the segmental resection group (15 of 37 versus three of 35, chi 2 = 8.17, 1 d.f., P = 0.004). Segmental resection following intraoperative irrigation is the preferred option except when there is caecal perforation or if synchronous neoplasms are present in the colon, when subtotal colectomy is more appropriate.