Objectives: Long-term adrenocorticotropic therapy (LT-ACTH), which consisted of 2-4 weeks of daily injections of adrenocorticotropic hormone (ACTH) and subsequent months of weekly injections, was tried for relapsed West syndrome (WS) or other intractable epilepsies in small case reports. Our aim was to explore the efficacy of LT-ACTH for preventing WS relapse, as well as the prevalence of its adverse events. Methods: This is a retrospective, nationwide, multicenter case series of patients with WS who underwent LT-ACTH. Clinical information of the patients and protocol of LT-ACTH were collected from participating institutes in this study. We defined clinical response to ACTH as achievement of hypsarrhythmia and epileptic spasms resolution. Patients who responded to daily ACTH injections were identified and assessed whether they experienced WS relapse during/after the weekly ACTH injection period. The outcome was measured by the nonrelapse rate at 24 months after daily ACTH injections using the Kaplan-Meier method. Results: Clinical information of 16 children with WS was analyzed. The median age at LT-ACTH initiation was 14.5 months (range: 7-68 months). Thirteen (81%) patients had previously undergone conventional ACTH treatment. The LT-ACTH regimens comprised a median of 16 days of daily injections (range: 11-28 days) and 10 months of weekly injections (range: 3-22 months). Seven patients experienced WS relapse during/after subsequent weekly ACTH period, and the nonrelapse rate at 24 months after daily injections was estimated at 60.6% (95% confidence interval: 32.3%-80.0%). Height stagnation, hypertension, and irritability were observed; lethal adverse events were not reported. Significance: Our study firstly explored the efficacy of LT-ACTH for preventing WS relapse. LT-ACTH might be a treatment option for patients with relapsed or intractable WS; however, we note that our study is limited by its small sample size and the lack of an appropriate control group.