BACKGROUND Corpus callosum agenesis (CCA) is one of the most common congenital brain abnormalities, and is associated with neurodevelopmental and neuropsychiatric disorders. In CCA, defects in osmoregulation have been reported. This report presents a rare case of chronic hyponatremia associated with the syndrome of inappropriate antidiuresis (SIAD) in a woman with CCA. CASE REPORT A 41-year-old woman presented to the renal unit with symptomatic hyponatremia. In her past medical history, she had a four-year history of systemic arterial hypertension and Sjögren's syndrome, and a three-year history of systemic lupus erythematosus (SLE), which was treated with cyclophosphamide. She had CCA but with irregular neurological follow-up. During the previous eight years, her plasma sodium levels ranged from between 118-134 mEq/L. On this hospital admission, she had plasma hypo-osmolality, measured in milli-osmoles per kilogram of H2O (mOsm/kg H2O), of 251 mOsm/Kg H2O, and a urinary hyper-osmolality of 545 mOsm/Kg H2O, and increased level of plasma antidiuretic hormone (ADH) (1.8 pg/ml). Bone densitometry was consistent with osteoporosis. The patient remained asymptomatic during her hospital stay. Chronic hyponatremia associated with the SIAD was diagnosed. Water restriction and increased protein intake resulted in a partial improvement in the serum sodium level (128-134 mEq/L). The patient was discharged from the hospital with outpatient follow-up. CONCLUSIONS A rare case of chronic hyponatremia due to the SIAD associated with CCA is reported that demonstrates the importance of correct diagnosis, management, and clinical follow-up of the SIAD, including bone densitometry.