Breathalyzer and blood-alcohol results from drivers arrested for operating a motor vehicle while intoxicated and for related offenses were compared during a two-year period. Four hundred and four pairs of breath- and blood-alcohol results from specimens collected within 1 h of each other were studied. Blood-alcohol concentrations ranged from zero to 0.421% weight per volume (w/v). Breath-alcohol concentrations ranged from zero to 0.44 g/210 L. The mean Breathalyzer result was 0.16 g/210 L. The mean blood-alcohol result was 0.176% w/v. Compared to the blood-alcohol result, Breathalyzer results were lower by more than 0.01 g/210 L 61% of the time, within 0.01 g/210 L 33% of the time, and higher by more than 0.01 g/210 L 6% of the time.