Ten experienced licensed private pilots were trained for 8 hours on a flight simulator landing task. They each smoked a cigarette containing 19 mg of delta 9-tetrahydrocannabinol (THC), and 24 hours later their mean performance on the flight task showed trends toward impairment on all variables, with significant impairment in number and size of aileron changes, size of elevator changes, distance off center on landing, and vertical and lateral deviation on approach to landing. Despite these deficits, the pilots reported no awareness of impaired performance. These results may have implications for performance of complex tasks the day after smoking marijuana.