Zolpidem is a widely used ultrashort-acting non-benzodiazepine in clinical practice; compared with benzodiazepines, it does not have side effects such as daytime hangover, rebound insomnia, and development of tolerance. We report an autopsy case of abnormal behaviour induced by zolpidem. A man in his 60's had suffered from postherpetic neuralgia about 2 months ago and had been prescribed zolpidem for insomnia. According to his family, he had no memory of his actions such as striking a wall, taking his futon outside, and eating 5 times a day after he took zolpidem. Because his postherpetic neuralgia did not improve, he was hospitalized and treated with an epidural block. During hospitalization, he took off his clothes, removed the epidural block catheter by himself, and slept on others' beds. He disappeared from the hospital one day; the next day, he was found dead in a narrow water storage tank 10 km away from the hospital. He was thought to have driven a car by himself to reach the place. Forensic autopsy revealed that the cause of death was drowning. Zolpidem and several other drugs were detected by toxicological analysis of his blood; the concentrations of these drugs were within therapeutic range. There are several reports about somnambulism induced by zolpidem such as sleepwalking, sleep driving, and eating. Considering the strange episodes following zolpidem administration, his behaviour on the day of his death was considered abnormal behaviour induced by zolpidem.