Esthesioneuroblastoma (ENB) is a rare tumor of the olfactory mucosa. We treated a 50-year-old man with an ENB in the right ethmoid sinus who had been diagnosed 16 years earlier with syndrome of inappropriate antidiuretic hormone secretion (SIADH) of unknown cause. When the ENB was surgically removed, the patient's osmoregulation returned to normal-that is, his SIADH resolved completely, which suggested that the SIADH was paraneoplastic in nature. These events prompted us to review the literature to determine if there is an association between our patient's ENB and his SIADH in general and between long-standing SIADH that precedes ENB in particular. Based on our review and an extrapolation of data, we have estimated that 1,300 cases of ENB have occurred since it was first described in 1924. Of these cases, SIADH was reported in 26 cases, including ours, which represents an estimated prevalence of 2% (although we believe this is actually an underestimation of the true prevalence). Of the 26 cases, SIADH had already been present in 14 patients (54%) prior to their diagnosis of EBN for a median duration of 3.5 years. We recommend that patients with newly diagnosed EBN be evaluated for SIADH. In those who are SIADH-positive, a resolution of SIADH should be expected once the ENB has been removed. If this does not occur, one should suspect that the ENB was not completely removed. If SIADH resolves but later recurs during follow-up, then a relapse should be suspected. In long-standing SIADH of unknown etiology, nasal sinus imaging should be considered.