BACKGROUND: The place of tonsillectomy in the management of sore throat in adults remains uncertain. OBJECTIVES: To establish the clinical effectiveness and cost-effectiveness of tonsillectomy, compared with conservative management, for tonsillitis in adults, and to evaluate the impact of alternative sore throat patient pathways. DESIGN: This was a multicentre, randomised controlled trial comparing tonsillectomy with conservative management. The trial included a qualitative process evaluation and an economic evaluation. SETTING: The study took place at 27 NHS secondary care hospitals in Great Britain. PARTICIPANTS: A total of 453 eligible participants with recurrent sore throats were recruited to the main trial. INTERVENTIONS: Patients were randomised on a 1 : 1 basis between tonsil dissection and conservative management (i.e. deferred surgery) using a variable block-stratified design, stratified by (1) centre and (2) severity. MAIN OUTCOME MEASURES: The primary outcome measure was the total number of sore throat days over 24 months following randomisation. The secondary outcome measures were the number of sore throat episodes and five characteristics from Sore Throat Alert Return, describing severity of the sore throat, use of medications, time away from usual activities and the Short Form questionnaire-12 items. Additional secondary outcomes were the Tonsil Outcome Inventory-14 total and subscales and Short Form questionnaire-12 items 6 monthly. Evaluation of the impact of alternative sore throat patient pathways by observation and statistical modelling of outcomes against baseline severity, as assessed by Tonsil Outcome Inventory-14 score at recruitment. The incremental cost per sore throat day avoided, the incremental cost per quality-adjusted life-year gained based on responses to the Short Form questionnaire-12 items and the incremental net benefit based on costs and responses to a contingent valuation exercise. A qualitative process evaluation examined acceptability of trial processes and ramdomised arms. RESULTS: There was a median of 27 (interquartile range 12-52) sore throats over the 24-month follow-up. A smaller number of sore throats was reported in the tonsillectomy arm [median 23 (interquartile range 11-46)] than in the conservative management arm [median 30 (interquartile range 14-65)]. On an intention-to-treat basis, there were fewer sore throats in the tonsillectomy arm (incident rate ratio 0.53, 95% confidence interval 0.43 to 0.65). Sensitivity analyses confirmed this, as did the secondary outcomes. There were 52 episodes of post-operative haemorrhage reported in 231 participants undergoing tonsillectomy (22.5%). There were 47 re-admissions following tonsillectomy (20.3%), 35 relating to haemorrhage. On average, tonsillectomy was more costly and more effective in terms of both sore throat days avoided and quality-adjusted life-years gained. Tonsillectomy had a 100% probability of being considered cost-effective if the threshold for an additional quality-adjusted life year was £20,000. Tonsillectomy had a 69% probability of having a higher net benefit than conservative management. Trial processes were deemed to be acceptable. Patients who received surgery were unanimous in reporting to be happy to have received it. LIMITATIONS: The decliners who provided data tended to have higher Tonsillectomy Outcome Inventory-14 scores than those willing to be randomised implying that patients with a higher burden of tonsillitis symptoms may have declined entry into the trial. CONCLUSIONS: The tonsillectomy arm had fewer sore throat days over 24 months than the conservative management arm, and had a high probability of being considered cost-effective over the ranges considered. Further work should focus on when tonsillectomy should be offered. National Trial of Tonsillectomy IN Adults has assessed the effectiveness of tonsillectomy when offered for the current UK threshold of disease burden. Further research is required to define the minimum disease burden at which tonsillectomy becomes clinically effective and cost-effective. TRIAL REGISTRATION: This trial is registered as ISRCTN55284102. FUNDING: This award was funded by the National Institute for Health and Care Research (NIHR) Health Technology Assessment programme (NIHR award ref: 12/146/06) and is published in full in Health Technology Assessment; Vol. 27, No. 31. See the NIHR Funding and Awards website for further award information.