To evaluate the diagnosis and treatment of syndrome of inappropriate antidiuresis ( SIAD ) in multiple system atrophy ( MSA ) . Clinical features and treatment of a case of MSA with SIAD was retrospectively analyzed. An MSA-cerebellar predominant ( MSA-C) type patient was admitted for being fever. Pulmonary and urinary infections were identified. The patient was characterized by euvolaemic hyponatremia with reduced plasma osmolality and increased urine sodium. Hypothyroidism and hypoadrenalism were excluded via functional tests of different hormones. The diagnosis of SIAD was made. The fever and SIAD persisted after pulmonary and urinary infections were cured. The patient was sweatless and the body temperature varied with room temperature due to dysfunction of central thermoregulation. With fluid restriction, the serum sodium return to the lower limit of normal range. Though the SIAD in MSA is rare, water and sodium balance should be carefully maintained, especially for those with pulmonary infection.