Telephone triage is proposed as a method of managing increasing demand for primary care. Previous studies have involved small samples in limited settings, and focused on nurse roles. Evidence is limited regarding the impact on primary care workload, costs, and patient safety and experience when triage is used to manage patients requesting same-day consultations in general practice. In comparison with usual care (UC), to assess the impact of GP-led telephone triage (GPT) and nurse-led computer-supported telephone triage (NT) on primary care workload and cost, patient experience of care, and patient safety and health status for patients requesting same-day consultations in general practice. Pragmatic cluster randomised controlled trial, incorporating economic evaluation and qualitative process evaluation. General practices (n = 42) in four regions of England, UK (Devon, Bristol/Somerset, Warwickshire/Coventry, Norfolk/Suffolk). Patients requesting same-day consultations. Practices were randomised to GPT, NT or UC. Data collection was not blinded; however, analysis was conducted by a statistician blinded to practice allocation. Primary - primary care contacts [general practice, out-of-hours primary care, accident and emergency (A&E) and walk-in centre attendances] in the 28 days following the index consultation request. Secondary - resource use and costs, patient safety (deaths and emergency hospital admissions within 7 days of index request, and A&E attendance within 28 days), health status and experience of care. Of 20,990 eligible randomised patients (UC n = 7283; GPT n = 6695; NT n = 7012), primary outcome data were analysed for 16,211 patients (UC n = 5572; GPT n = 5171; NT n = 5468). Compared with UC, GPT and NT increased primary outcome contacts (over 28-day follow-up) by 33% [rate ratio (RR) 1.33, 95% confidence interval (CI) 1.30 to 1.36] and 48% (RR 1.48, 95% CI 1.44 to 1.52), respectively. Compared with GPT, NT was associated with a marginal increase in primary outcome contacts by 4% (RR 1.04, 95% CI 1.01 to 1.08). Triage was associated with a redistribution of primary care contacts. Although GPT, compared with UC, increased the rate of overall GP contacts (face to face and telephone) over the 28 days by 38% (RR 1.38, 95% CI 1.28 to 1.50), GP face-to-face contacts were reduced by 39% (RR 0.61, 95% CI 0.54 to 0.69). NT reduced the rate of overall GP contacts by 16% (RR 0.84, 95% CI 0.78 to 0.91) and GP face-to-face contacts by 20% (RR 0.80, 95% CI 0.71 to 0.90), whereas nurse contacts increased. The increased rate of primary care contacts in triage arms is largely attributable to increased telephone contacts. Estimated overall patient-clinician contact time on the index day increased in triage (GPT = 10.3 minutes; NT = 14.8 minutes; UC = 9.6 minutes), although patterns of clinician use varied between arms. Taking account of both the pattern and duration of primary outcome contacts, overall costs over the 28-day follow-up were similar in all three arms (approximately £75 per patient). Triage appeared safe, and no differences in patient health status were observed. NT was somewhat less acceptable to patients than GPT or UC. The process evaluation identified the complexity associated with introducing triage but found no consistency across practices about what works and what does not work when implementing it. Introducing GPT or NT was associated with a redistribution of primary care workload for patients requesting same-day consultations, and at similar cost to UC. Although triage seemed to be safe, investigation of the circumstances of a larger number of deaths or admissions after triage might be warranted, and monitoring of these events is necessary as triage is implemented. Current Controlled Trials ISRCTN20687662. This project was funded by the NIHR Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 19, No. 13. See the NIHR Journals Library website for further project information.