BACKGROUND: Esthesioneuroblastoma is an uncommon intranasal tumor. These neuroendocrine neoplasms are rarely associated with excess hormone syndromes, and only nine cases of inappropriate antidiuretic hormone syndrome (SIADH) secondary to an esthesioneuroblastoma have been described. In all these cases, electrolyte abnormalities were seen when the tumor initially developed. We report a unique case of esthesioneuroblastoma recurrence manifesting as SIADH as the solely presenting feature. METHODS AND RESULTS: A 34-year-old woman was referred to us for evaluation for hyponatremia. She had undergone resection of an esthesioneuroblastoma at age 18 with radiotherapy. The patient had undergone annual CT of the nasal area and was considered to be disease free. The study of the hyponatremia was consistent with a SIADH, and MRI revealed an intranasal mass. The resection of the tumor reversed the hyponatremia, and pathologic analysis revealed a recurrence of esthesioneuroblastoma. CONCLUSIONS: Biochemical analysis should be performed in the follow-up of patients with esthesioneuroblastoma. In our case, the biochemical abnormality led to the diagnosis of the recurrence.