The aim of the study is to assess if cognitive impairments in abstinent users of methylenedioxymethamphetamine (MDMA) are due to repeated use of MDMA or concurrent drugs of abuse and how these impairments interfere with key activities of daily life such as automobile driving. Simulated driving performance and attention of 12 users of MDMA and marijuana [tetrahydrocannabinol (THC)] (MDMA/THC), 15 THC users, and 15 non-drug-using, age-matched controls were compared. THC users' mean speed reduction in response to passing a car parked on the shoulder of the road was 7 mph (11.3 km/h) less than that of controls. Compared with non-drug-using controls, the MDMA/THC users were less able to mitigate the effects of an illegal intersection incursion by another vehicle and entered the collision at a higher speed (42 versus 26 mph). In real life, this 16-mph (26-km/h) difference would increase the likelihood of injuring parties in both colliding vehicles. Processing speed and attention did not differ between groups; this suggests that the lower speed reduction in the MDMA/THC users was more likely due to executive function impairments (incorrect situation interpretation and awareness) than to reduced processing speed. In conclusion, compared with non-drug users, MDMA/THC users may be at a greater risk for driver performance errors that lead to crashes. Because collision speed did not differ between MDMA/THC users and THC users, a specific role of MDMA as opposed to THC toxicity in driving performance impairment remains to be established.