We describe the first reported case of a syndrome of inappropriate secretion of antidiuretic hormone (SIADH) induced by low-dose tacrolimus in a patient with autoimmune disease. A 41-year-old man with systemic lupus erythematosus (SLE) developed hyponatremia induced by SIADH after administration of tacrolimus (0.06 mg/kg per day). In this case, the hyponatremia promptly resolved upon withdrawal of tacrolimus. This case strongly suggests that SIADH is a potentially important complication of tacrolimus administration, irrespective of dosage, and should be borne in mind whenever the drug is used.