Recent policy changes in California regarding cannabis use underscore the need to study outcomes and prevalence of this drug in trauma. Our study aims to study the prevalence of cannabis use and associations with injury types and outcomes in Los Angeles County trauma patients. Data were reviewed from 21,276 adult patients from a Los Angeles countywide database spanning five years (2012-2016), who underwent urine toxicology testing in the ED after sustaining a traumatic injury. The percentage of trauma patients using marijuana increased from 36 to 43 per cent over the five-year period. On univariate analysis, cannabis-positive patients were significantly younger and more likely male, with lower median systolic blood pressure and heart rate on arrival in the ED. A higher proportion of cannabis users had penetrating trauma, and 48 per cent of cannabis users also tested positive for amphetamines, cocaine, opioids, or Phencyclidine. On multivariate analysis, cannabis was associated with an increase in need for mechanical ventilation after adjusting for age, admission Glasgow Coma Score, gender, polysubstance use, blunt or penetrating mechanism, and Injury Severity Score, and was not associated with increases in mortality or ICU length of stay.