Recurrent hallucinations appeared in an 11-year-old boy during 5 days following ketamine anaesthesia. Previous anaesthesia with ketamine and adequate diazepam supplementation did not produce any such effect. The phenomenon of delayed recurring hallucinations is a rare but dangerous side-effect of ketamine, not unlike LSD flashbacks. The described case lends support to previous reports on the value of diazepam in the prevention of post-ketamine perceptual abnormalities.