Background and purposeA single, oral dose of 3 mg/day tacrolimus, approved for myasthenia gravis (MG) treatment in Japan, was shown to reduce steroid dose and anti‐acetylcholine receptor (AChR) antibody titers as well as to improve MG symptoms. However, no studies have investigated the association between tacrolimus concentration and its clinical efficacy in MG. In this study, we aimed to determine the optimal tacrolimus concentration for MG treatment.MethodsThe trough tacrolimus concentration in 51 patients with MG (positive for anti‐AChR antibody, n = 48; negative for anti‐AChR and anti‐muscle‐specific tyrosine kinase antibodies, n = 3) who received 3 mg/day tacrolimus for more than 1 year was measured using a chemiluminescent enzyme immunoassay. The clinical characteristics of patients with MG as well as the dose of prednisolone used before and after tacrolimus treatment were evaluated retrospectively.ResultsThe median trough tacrolimus concentration was 5.4 (range, 2.9–7.6) ng/mL, which was correlated with ‘minimal manifestation or better status’ (P = 0.0190, r = 0.3273) and the reduction in anti‐AChR antibody 1 year after tacrolimus initiation (P = 0.0170, r = 0.3465). When the cut‐off value for tacrolimus was defined as 4.8 ng/mL using a receiver operating characteristic curve, patients with adequate tacrolimus concentration (≥4.8 ng/mL) showed more reduction in anti‐AChR antibody titers and more improvement in MG‐related activities in daily life scores. More patients with adequate tacrolimus concentration achieved ‘minimal manifestation or better status’ compared with those with low tacrolimus concentration.ConclusionsAn adequate tacrolimus concentration is required for better MG prognosis.