To reduce the involvement of young drivers in alcohol-related crashes, 29 States and the District of Columbia have established lower legal blood alcohol limits for drivers younger than age 21 than for adult drivers. Of these, 12 lowered the legal limit for young people prior to 1991. To assess the impact, these 12 States were paired for comparison with 12 nearby States matched for legal drinking age and timing of changes in that law. Among drivers ages 15-20, fatal crashes involving a single vehicle at night are three times more likely than other fatal crashes to be alcohol-related. Whether the proportion of fatal crashes that involved single vehicles at night declined more among young drivers targeted by lower blood alcohol limits than among young drivers of the same age in comparison States was examined. The maximum available equal number of pre- and post-law years were compared in each pair of States. During the post-law period, the proportion of fatal crashes that involved single vehicles at night declined 16 percent among young drivers targeted by lower blood alcohol laws, whereas it rose 1 percent among drivers of the same age in comparison States where blood alcohol limits were not changed (P < .001). Among adults, the proportion of fatal nocturnal crashes that involved single vehicles declined 5 percent in the group of States with the lowered levels for young people during the period after the law was enacted and 6 percent in the group of neighboring comparison States. The proportion of fatal crashes that involved single vehicles at night declined 22 percent among drivers in States with .00 percent limits, whereas it declined only 2 percent among drivers of the same age in comparison States (P <.003). Among those targeted by .02 percent BAL limits, the proportion of fatal crashes that involved single vehicles at night declined 17 percent. It rose 4 percent in comparison States(P = .005). No significant difference appeared between States that lowered blood alcohol levels to the range of .04-.06 percent relative to comparison States.If all States adopted .00 or .02 percent limits for drivers ages 15-20, at least 375 fatal single vehicle crashes at night would be prevented each year.