Opioid-induced rigidity is a potentially life-threatening complication that can occur after treatment with large doses of opioids, but with early recognition it can be treated effectively with naloxone or with muscle relaxants. Regarding its onset time, there have been few case reports that have described delayed manifestations of opioid-induced rigidity. The mechanism of this complication is not well understood. In this report we describe a case of incidental overdose injection of sufentanil and subsequently review the confusing clinical features that require immediate diffenrentiation and the possible mechanim of this complication. (Korean J Pain 2008; 21: 66?70)