BACKGROUND: Systemic adjuvant treatment might mitigate the high risk of disease recurrence in patients with resected stage IIC-III melanoma. The BRIM8 study evaluated adjuvant vemurafenib monotherapy in patients with resected, BRAF(V600) mutation-positive melanoma. METHODS: BRIM8 was a phase 3, international, double-blind, randomised, placebo-controlled study that enrolled 498 adults (aged >/=18 years) with histologically confirmed stage IIC-IIIA-IIIB (cohort 1) or stage IIIC (cohort 2) BRAF(V600) mutation-positive melanoma that was fully resected. Patients were randomly assigned (1:1) by an interactive voice or web response system to receive twice-daily adjuvant oral vemurafenib 960 mg tablets or matching placebo for 52 weeks (13 x 28-day cycles). Randomisation was done by permuted blocks (block size 6) and was stratified by pathological stage and region in cohort 1 and by region in cohort 2. The investigators, patients, and sponsor were masked to treatment assignment. The primary endpoint was disease-free survival in the intention-to-treat population, evaluated separately in each cohort. Hierarchical analysis of cohort 2 before cohort 1 was prespecified. This trial is registered with ClinicalTrials.gov, number NCT01667419. FINDINGS: The study enrolled 184 patients in cohort 2 (93 were assigned to vemurafenib and 91 to placebo) and 314 patients in cohort 1 (157 were assigned to vemurafenib and 157 to placebo). At the time of data cutoff (April 17, 2017), median study follow-up was 33.5 months (IQR 25.9-41.6) in cohort 2 and 30.8 months (25.5-40.7) in cohort 1. In cohort 2 (patients with stage IIIC disease), median disease-free survival was 23.1 months (95% CI 18.6-26.5) in the vemurafenib group versus 15.4 months (11.1-35.9) in the placebo group (hazard ratio [HR] 0.80, 95% CI 0.54-1.18; log-rank p=0.026). In cohort 1 (patients with stage IIC-IIIA-IIIB disease) median disease-free survival was not reached (95% CI not estimable) in the vemurafenib group versus 36.9 months (21.4-not estimable) in the placebo group (HR 0.54 [95% CI 0.37-0.78]; log-rank p=0.0010); however, the result was not significant because of the prespecified hierarchical prerequisite for the primary disease-free survival analysis of cohort 2 to show a significant disease-free survival benefit. Grade 3-4 adverse events occurred in 141 (57%) of 247 patients in the vemurafenib group and 37 (15%) of 247 patients in the placebo group. The most common grade 3-4 adverse events in the vemurafenib group were keratoacanthoma (24 [10%] of 247 patients), arthralgia (17 [7%]), squamous cell carcinoma (17 [7%]), rash (14 [6%]), and elevated alanine aminotransferase (14 [6%]), although all keratoacanthoma events and most squamous cell carcinoma events were by default graded as grade 3. In the placebo group, grade 3-4 adverse events did not exceed 2% for any of the reported terms. Serious adverse events were reported in 40 (16%) of 247 patients in the vemurafenib group and 25 (10%) of 247 patients in the placebo group. The most common serious adverse event was basal cell carcinoma, which was reported in eight (3%) patients in each group. One patient in the vemurafenib group of cohort 2 died 2 months after admission to hospital for grade 3 hypertension; however, this death was not considered to be related to the study drug. INTERPRETATION: The primary endpoint of disease-free survival was not met in cohort 2, and therefore the analysis of cohort 1 showing a numerical benefit in disease-free survival with vemurafenib versus placebo in patients with resected stage IIC-IIIA-IIIB BRAF(V600) mutation-positive melanoma must be considered exploratory only. 1 year of adjuvant vemurafenib was well tolerated, but might not be an optimal treatment regimen in this patient population. FUNDING: F Hoffman-La Roche Ltd.