ObjectivesTo evaluate the long-term outcome of tacrolimus for childhood-onset myasthenia gravis (CMG) with an inadequate response to glucocorticoids, and investigate factors associated with favorable outcomes following tacrolimus treatment. MethodsA retrospective, observational cohort study was performed for CMG patients who had not improved satisfactorily after sufficient prednisone therapy for at least 8 weeks. All patients were given tacrolimus in doses of 2-3 mg for more than 6 months. The primary efficacy outcome was assessed using the prednisone dose, quantitative MG (QMG), and MG-activity of daily living (ADL) scores. The participants were divided into improved and unimproved groups based on changes in QMG scores to investigate the risk factors that affected tacrolimus efficacy. ResultsA total of 149 glucocorticoid resistant CMG patients were finally enrolled in our study, with 113 (75.8%) responding well to tacrolimus (defined as minimal manifestation status or better). One month after initiating tacrolimus, there was a noticeable improvement in prednisone dose, QMG, and ADL scores, which continued to improve throughout the study. More importantly, the prednisone was eventually stopped in 89 of the patients (78.8%). Thymus type [odds ratio (OR) = 3.156, 95% confidence interval (CI) 1.427-6.978; P = 0.005] and pre-intervention status (OR = 0.284, 95%CI 0.109-0.741; P = 0.010) were independent predictors of tacrolimus efficacy after controlling for confounding factors in multiple logistic regression. ConclusionThe majority of glucocorticoid-resistant CMG patients have a good long-term prognosis after adding tacrolimus. Thymus type and pre-intervention status can serve as potential predictors affecting the efficacy of tacrolimus.