Carbon monoxide in concentrations above 15% in the blood appears only rarely to be a factor in highway crashes. However, carbon monoxide in lower concentrations, and cigarette smoking which can produce such concentrations, have been hypothesized as important hazards to safe driving and one study has reported that smokers have higher crash rates than do non-smokers. In the present study, however, fatally injured drivers had evidence of heavy smoking more often than did drivers not involved in crashes but stopped at roadblocks at times and places where crashes had occurred. But the entire excess was attributable to the fact that heavy smokers also are heavy drinkers, and often are problem drinkers. Persons who smoke heavily but do not drink heavily were not over-represented among the fatalities. It is hypothesized that problem drinkers represent a substantial proportion of persons with chronic respiratory disease just as they do among highway fatalities.