AbstractBackground Studies looking at the association between perceived risk and simultaneous use of alcohol and marijuana are scarce. The present study has three purposes: (1) to examine the association between alcohol and marijuana use at the daily level; (2) to document how this association varies by the perceived risk of using alcohol and marijuana simultaneously; (3) to test whether the association varies by college attendance. Methods 89 young adults (Mean Age = 18.3 years, SD = 0.5) participated between October 2012 and May 2013. Participants completed a 30-min survey followed by 14 brief daily surveys in each of three waves. Results Alcohol use on a given day was associated with increased odds of marijuana use that day, especially among young adults with lower perceived risk. For college students, the association between alcohol and marijuana was weaker than for non-students. Conclusions Alcohol and marijuana use were associated at a daily level, especially among young adults with lower perceived risk and those who were not attending college.