Researchers performed comprehensive drug screens on blood specimens collected from 168 fatally injured drivers. One or more drugs were detected in 67 percent of the drivers, and 33 percent of the drivers had detectable blood concentrations of psychoactive drugs or alcohol. The most prevalent drugs found were cannabinoids and ethanol, each found in 13 percent of the drivers. Cocaine or benzoylecgonine was found in 8 percent of the cases. Seven percent of the drivers' blood specimens contained amphetamine or methamphetamine, and 7 percent contained phenylpropanolamine, ephedrine, or pseudoephedrine. A panel of toxicologists reviewed the accident investigation report and the toxicology findings for each case. The panel decided that impairment due to marijuana use was a factor always where the delt-9-tetrahydrocannabinaol concentration exceeded 1.0 ng/mL and that alcohol impairment contributed to all accidents where the blood alcohol concentration was 0.04 wt/vol or greater. In 50 of 56 cases where psychoactive drugs or alcohol were found, impairment due to substance use contributed to the fatal accident. 5 tables and 38 references