The final purpose of oral & maxillofacial trauma is functional 4 esthetic repair. Nowadays, severe trauma involving with the head & neck trauma is increasing. After these trauma occurs, the patients develop similar signs & symptoms with the postoperative healing period, as like thurst, hypertention, excitability, disorientation, convulsion, et al. Because SIADH which is one of important complications after head trauma, shows similar clinical features after operation, we should pay attention to detect it. SIADH shows characteristic laboratory findings, as like kyponatremia, urine kyperosmolality, increased plasma ADH level, continued renal excretion of sodium, so we can easily distinguish it from postoperative conditions. This paper reports two cases, one was the case of the mandibular fracture and cerebral contusion, which included permanent SIADH. The other was the case of the multiple teeth injury and cerebral contusion, which was transient SIADH. We treated them with water restriction, hypertonic saline, and diuretics.