Tefft BC, et al. - The present study was conducted to calculate cannabis involvement in fatal crashes in Washington state before and after recreational cannabis legalization using multiple imputation of missing values Researchers applied data on all drivers involved in fatal crashes in Washington in years 2008–2019 (n=8,282) to calculate prevalence in fatal crashes of drivers with ∆9-tetrahydrocannabinol (THC, the main psychoactive compound in cannabis) in their blood before and after legalization. Logistic regression followed by marginal standardization was used to calculate the adjusted prevalence of THC-positive drivers after legalization relative to what would have been predicted without legalization. The results demonstrate that some of the increased prevalence of THC-positive drivers might have reflected cannabis use unassociated with driving; nevertheless, the elevated prevalence of drivers with high THC concentrations implies increased prevalence of driving shortly after using cannabis. Moreover, other jurisdictions should compile quantitative data on drug test results of drivers to enable surveillance and assessment.