Toxicological determinations are crucial to coroners' or medical examiners' judgments that drugs are significantly involved in a death. However, differences in laboratory procedures, thoroughness of screening, and limits of detection may result in artifactual differences in the toxicological results and the subsequent interpretations of them. To test this possibility, we conducted a toxicology proficiency-testing survey of nine collaborating laboratories. The results for the proficiency samples point out starting interlaboratory differences in accuracy and precision of detection of drugs. These observed variations in toxicological proficiency may introduce a significant source of error in drug-death statistics and in epidemiological deductions based on these statistics.