© 2017 American Medical Association. All rights reserved. IMPORTANCE The histiocytic neoplasms Erdheim-Chester disease (ECD) and Langerhans cell histiocytosis (LCH) are highly enriched for BRAF V600 mutations and have been previously shown to be responsive to treatment with vemurafenib, an inhibitor of the BRAF V600 kinase. However, the long-term efficacy and safety of prolonged vemurafenib use in these patients are not defined. Here we analyze the final efficacy and safety data for vemurafenib in patients with ECD and LCH enrolled in the VE-BASKET study. OBJECTIVE To determine the efficacy and safety of vemurafenib in adults with ECD or LCH enrolled in the VE-BASKET study. DESIGN, SETTING, AND PARTICIPANTS The VE-BASKET study was an open-label, nonrandomized, multicohort study for patients with nonmelanoma cancers harboring the BRAF V600 mutation. Patients with BRAF V600-mutant ECD or LCH were enrolled in an “other solid tumor” cohort of the VE-BASKET study, and they were enrolled in the present study. INTERVENTIONS Patients received vemurafenib, 960 mg, twice daily continuously until disease progression, study withdrawal, or occurrence of intolerable adverse effects. MAIN OUTCOMES AND MEASURES The primary end point was confirmed objective response rate (ORR) by Response Evaluation Criteria in Solid Tumors (RECIST, version 1.1). Secondary end points included progression-free survival (PFS), overall survival (OS), metabolic response by modified positron-emission tomography (PET) Response Criteria in Solid Tumors (PERCIST) using18F-fluorodeoxyglucose (FDG)-PET/computed tomography (CT), and safety. RESULTS A total of 26 patients from the VE-BASKET trial (22 with ECD, 4 with LCH) were included in the present study (14 women and 12 men; median age, 61 years; age range, 51-74 years). The confirmed ORR was 61.5% (95% CI, 40.6%-79.8%) in the overall cohort and 54.5% (95% CI, 32.2%-75.6%) in patients with ECD. All evaluable patients achieved stable disease or better. The median PFS and OS had not been reached in the overall cohort at study closure despite a median follow-up of 28.8 months; 2-year PFS was 86% (95% CI, 72%-100%), and 2-year OS was 96% (95% CI, 87%-100%). All 15 patients evaluated by FDG-PET/CT achieved a metabolic response, including 12 patients (80%) with a complete metabolic response. The most common adverse events (AEs) in the overall cohort included arthralgia, maculopapular rash, fatigue, alopecia, prolonged QT interval, skin papilloma, and hyperkeratosis. Hypertension and dermatologic AEs occurred at higher rates than those reported in metastatic melanoma. CONCLUSIONS AND RELEVANCE In this study, vemurafenib had prolonged efficacy in patients with BRAF V600-mutant ECD and LCH and warrants consideration as a new standard of care for these patients.