The optimal treatment strategy for adult Langerhans cell histiocytosis (LCH) remains unclear. Our previous study demonstrated the remarkable efficacy of combined methotrexate and cytarabine (Ara-C) [MA] therapy in patients newly diagnosed with LCH, with a median follow-up of 2 years. The present article reports long-term follow-up data spanning a median of 78 months (6.5 years) from a single-arm, single-centre, prospective phase 2 clinical trial (NCT02389400) conducted between January 2014 and December 2020. Ninety-five adults with newly diagnosed LCH exhibiting multisystem disease or multifocal single-system involvement underwent MA therapy every 35 days for six cycles. Methotrexate (1 g/m²) was administered by 24 h infusion on day 1 and AraC (0.1 g/m²) by 24 h infusion for 5 days. The primary end-point was event-free survival (EFS). The median patient age was 32 years (range 18-65 years). The overall response rate was 89.5%. Seven patients in this cohort died, and 38 experienced disease reactivation. No degenerative central nervous system diseases were observed. The estimated 6-year overall survival (OS) and EFS rates were 93.2% and 55.2% respectively. Multivariate analysis revealed that risk organ (RO) involvement at baseline (hazard ratio [HR] 6.135 [95% confidence interval (CI) 1.185-32.259]; p = 0.031) and age >40 years at diagnosis (HR 7.299 [95% CI 1.056-21.277]; p = 0.042) were associated with inferior OS. RO (HR 2.604 [95% CI 1.418-4.762]; p = 0.002) and skin (HR 2.232 [95% CI 1.171-4.255]; p = 0.015) involvement at baseline were poor prognostic factors for EFS. Regarding adverse events, four patients developed a second primary malignancy. In conclusion, the MA regimen was a valid and safe therapeutic approach for adult patients newly diagnosed with LCH.