We reported a case of considerable decrease in tacrolimus blood trough level after the intake of mung bean (Vigna radiata L.) soup in a patient with dermatomyositis. A 33-year-old woman, diagnosed with classic dermatomyositis, received treatment with prednisolone and tacrolimus. The blood concentration of tacrolimus decreased by ~50%. Mung bean soup was suspected as the factor that might affect the elimination of tacrolimus. It is possible that the induction of CYP3A enzymes and/or P-gp, mediated by flavonoids in the mung bean, contributed to the marked decrease of tacrolimus blood trough level. And both clinicians and pharmacists should be aware of the possible food-drug interaction between mung bean food and tacrolimus.