Background Robotic rectal cancer surgery is gaining popularity, but there are limited data about its safety and efficacy. Objective To undertake an evaluation of robotic compared with laparoscopic rectal cancer surgery to determine its safety, efficacy and cost-effectiveness. Design This was a multicentre, randomised trial comparing robotic with laparoscopic rectal resection in patients with rectal adenocarcinoma. Setting The study was conducted at 26 sites across 10 countries and involved 40 surgeons. Participants The study involved 471 patients with rectal adenocarcinoma. Recruitment took place from 7 January 2011 to 30 September 2014 with final follow-up on 16 June 2015. Interventions Robotic and laparoscopic rectal cancer resections were performed by high anterior resection, low anterior resection or abdominoperineal resection. There were 237 patients randomised to robotic and 234 to laparoscopic surgery. Follow-up was at 30 days, at 6 months and annually until 3 years after surgery. Main outcome measures The primary outcome was conversion to laparotomy. Secondary end points included intra- and postoperative complications, pathological outcomes, quality of life (QoL) [measured using the Short Form questionnaire-36 items version 2 (SF-36v2) and the Multidimensional Fatigue Inventory-20 (MFI-20)], bladder and sexual dysfunction [measured using the International Prostatic Symptom Score (I-PSS), the International Index of Erectile Function (IIEF) and the Female Sexual Function Index (FSFI)], and oncological outcomes. An economic evaluation considered the costs of robotic and laparoscopic surgery, including primary and secondary care costs up to 6 months post operation. Results Among 471 randomised patients [mean age 64.9 years, standard deviation (SD) 11.0 years; 320 (67.9%) men], 466 (98.9%) patients completed the study. Data were analysed on an intention-to-treat basis. The overall rate of conversion to laparotomy was 10.1% and occurred in 19 (8.1%) patients in the robotic-assisted group and in 28 (12.2%) patients in the conventional laparoscopic group {unadjusted risk difference 4.12% [95% confidence interval (CI) –1.35% to 9.59%], adjusted odds ratio 0.61 [95% CI 0.31 to –1.21]; p = 0.16}. Of the nine prespecified secondary end points, including circumferential resection margin positivity, intraoperative complications, postoperative complications, plane of surgery, 30-day mortality and bladder and sexual dysfunction, none showed a statistically significant difference between the groups. No difference between the treatment groups was observed for longer-term outcomes, disease-free and overall survival (OS). Males were at a greater risk of local recurrence than females and had worse OS rates. The costs of robotic and laparoscopic surgery, excluding capital costs, were £11,853 (SD £2940) and £10,874 (SD £2676) respectively. Conclusions There is insufficient evidence to conclude that robotic rectal surgery compared with laparoscopic rectal surgery reduces the risk of conversion to laparotomy. There were no statistically significant differences in resection margin positivity, complication rates or QoL at 6 months between the treatment groups. Robotic rectal cancer surgery was on average £980 more expensive than laparoscopic surgery, even when the acquisition and maintenance costs for the robot were excluded. Future work The lower rate of conversion to laparotomy in males undergoing robotic rectal cancer surgery deserves further investigation. The introduction of new robotic systems into the market may alter the cost-effectiveness of robotic rectal cancer surgery. Trial registration Current Controlled Trials ISRCTN80500123. Funding This project was funded by the Efficacy and Mechanism Evaluation (EME) programme, a Medical Research Council and National Institute for Health Research (NIHR) partnership, with contributions from the Chief Scientist Office, Scottish Government Health and Social Care Directorate, the Health and Care Research Wales and the Health and Social Care Research and Development Division, Public Health Agency in Northern Ireland. The funders of the study had no role in the design and conduct of the study; collection, management, analysis and interpretation of the data; and preparation, review or approval of the manuscript or the decision to submit for publication. The project will be published in full in Efficacy and Mechanism Evaluation; Vol. 6, No. 10. See the NIHR Journals Library website for further project information. Philip Quirke and Nicholas West were supported by Yorkshire Cancer Research Campaign and the MRC Bioinformatics initiative. David Jayne was supported by a NIHR Research Professorship.