Traditional all-terrain vehicles (ATVs) are designed for single riders. Although carrying passengers is a known risk factor for injury, how passengers contribute to ATV crashes remains poorly understood. To address this question, we performed a retrospective chart review of ATV crash victims at a U.S. trauma center (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013). Of 537 cases, 20% were passengers or drivers with passengers. The odds of backward rollovers, falls/ejections, crashes on sloped terrain, and collisions with motorized vehicles were all significantly greater when passengers were present. In contrast, the odds of self-ejection or falls/ejections over the handlebars were significantly lower than falls/ejections to the side or rear, in crashes with multiple riders. Among all ejections, self-ejections had the lowest head and highest extremity injury scores and being ejected over the handlebars or to the rear resulted in worse head injury scores than being ejected to the side. In summary, our study found that passengers increased the odds of specific crash and injury mechanisms and that head and extremity injury severity varied by ejection type. Safety interventions including seat design changes that prevent carrying passengers, and a strict, well-enforced no-rider rule are needed to effectively prevent passenger-related deaths and injuries.