This study examined the impact of alcohol hangover on flight performance 11 hr after ingestion of alcohol. The participants were 36 instrument-rated and instrument-current male pilots who were 21 to 35 years of age, recruited from the Odegard Center for Aerospace Sciences at the University of North Dakota. Participants were randomly assigned to receive either a placebo or 2 or 3 ml/kg of 100 proof Vodka the evening before their flights were scheduled. After alcohol was administered, participants were provided sleeping accommodations and breakfast the next morning. Starting at 8 a.m. participants were asked to fly two 75-min scenarios on a Frasca 241 simulator. Performance deficits were observed on measures of rate of turn and bank for the group who had ingested the largest dose of ethanol. In addition, participants in this group correctly carried out fewer directives from the air traffic controller than those who received the placebo or the 2 ml/kg dose. The hangover-induced deficits observed may have been larger if participants were tested at a simulated altitude. The necessity of measuring biological indexes of hangover are discussed.