Purpose: The specific mechanisms by which peer passengers increase teen drivers' crash risk are not completely understood. We aimed to provide insight on the two primary hypothesized mechanisms, distraction and promotion of risk-taking behavior, for male and female teen drivers and further for select driver-passenger gender combinations. Methods: From the National Motor Vehicle Crash Causation Survey (2005-2007), we analyzed a nationally representative sample of 677 drivers aged 16-18 years (weighted n = 277,484) involved in serious crashes, to compare the risk of specific distraction-related and risk-taking-related precrash factors (documented via on-scene crash investigation) for teens driving with peer passengers and teens driving alone. Results: Compared with males driving alone, those with peer passengers were more likely to perform an aggressive act (risk ratio, RR [95% confidence interval] = 2.36 [1.29-4.32]) and perform an illegal maneuver (RR = 5.88 [1.81-19.10]) just before crashing; risk taking increased regardless of passenger gender. Crash-involved males with passengers were also more likely to be distracted by an exterior factor (RR = 1.70 [1.15-2.51]). Conversely, females with passengers were more often engaged in at least one interior nondriving activity (other than conversing with passengers) (RR = 3.87 [1.36-11.06]), particularly when driving with opposite-gender passengers. Female drivers, both with and without passengers, rarely drove aggressively or performed an illegal maneuver before crashing. Conclusions: Passengers may affect male teen driver crashes through both distraction and risk-promoting pathways, and female involvement primarily through internal distraction. Results of this and future studies investigating peer-driver interactions may guide development of passenger-related crash prevention efforts to complement already existing Graduated Driver Licensing passenger restrictions. © 2012 Society for Adolescent Health and Medicine.