A 40-year-old achondroplastic patient underwent posterior spinal fusion under general endotracheal anesthesia. Anesthesia was maintained with isoflurane, and sufentanil, dexmedetomidine, and lidocaine infusions. Urine output increased from 150 mL/hr to 950 mL/hr the fourth hour. An increasing serum sodium, low urine-specific gravity, and increased serum osmolarity occurred simultaneously with the polyuria. Within 2 hours of discontinuing the dexmedetomidine infusion urine output greatly decreased. Within 24 hours all signs of the polyuric syndrome resolved spontaneously. Alpha(2) agonists block arginine-vasopressin release and action; however, a polyuric syndrome has not been reported in the human literature.