Summary Background Laparoscopic-assisted surgery for colorectal cancer has been widely adopted without data from large-scale randomized trials to support its use. We compared short-term endpoints of conventional versus laparoscopic-assisted surgery in patients with colorectal cancer to predict long-term outcomes. Methods Between July, 1996, and July, 2002, we undertook a multicenter, randomized clinical trial in 794 patients with colorectal cancer from 27 UK centers. Patients were allocated to receive laparoscopic-assisted (n=526) or open surgery (n=268). Primary short-term endpoints were positivity rates of circumferential and longitudinal resection margins, proportion of Dukes' C2 tumors, and in-hospital mortality. Analysis was by intention to treat. This trial has been assigned the International Standard Randomized Controlled Trial Number ISRCTN74883561. Findings Six patients (two [open], four [laparoscopic]) had no surgery, and 23 had missing surgical data (nine, 14). 253 and 484 patients actually received open and laparoscopic-assisted treatment, respectively. 143 (29%) patients underwent conversion from laparoscopic to open surgery. Proportion of Dukes' C2 tumors did not differ between treatments (18 [7%] patients, open vs 34 [6%], laparoscopic; difference -0·3%, 95% CI -3·9 to 3·4%, p=0·89), and neither did in-hospital mortality (13 [5%] vs 21 [4%]; -0·9%, -3·9 to 2·2%, p=0·57). Apart from patients undergoing laparoscopic anterior resection for rectal cancer, rates of positive resection margins were similar between treatment groups. Patients with converted treatment had raised complication rates. Interpretation Laparoscopic-assisted surgery for cancer of the colon is as effective as open surgery in the short term and is likely to produce similar long-term outcomes. However, impaired short-term outcomes after laparoscopic-assisted anterior resection for cancer of the rectum do not yet justify its routine use. [ABSTRACT FROM AUTHOR]