Peer mentoring has long been accepted as beneficial in a multitude of fields, but there has been limited description or study of the way these types of relationships may benefit medical students. Peer mentoring may be helpful for 1st-year medical students; however, little evidence is thus far available on which aspects of peer mentoring provide benefit and what those specific benefits are. This study examines the perceived benefits and satisfaction derived by 1st-year medical students from participation in a semi-structured, informal cross-year peer mentoring program. It further attempts to characterize the aspects of peer mentoring relationships that are seen as beneficial to 1st-year students. Data were collected on demographic information, frequency and type of contact with mentor, perception of mentor characteristics, perceived benefits derived from the relationship, and their overall satisfaction with the relationship. First-year medical students at a large academic institution were surveyed on their experience in an informal peer-mentoring program with 2nd-year students serving as mentors. Mentors had minimal training. No mandatory agenda or limitations were placed on the type or subject matter of interactions. Mean overall satisfaction with the peer mentoring program was 7.47 ( = 2.45) on a rating scale where 10 is the most satisfied. Students reported that their peer mentors generally exhibited beneficial attributes and behaviors. Ratings of perceived benefits were somewhat more variable and generally lower. Predictors of four outcomes-overall satisfaction, perceived academic benefit, perceived nonacademic benefit, and perceived clinical benefit-were explored using stepwise linear regression with forward entry and backward elimination to retain the most parsimonious model. Expertise in areas of mentee need was a consistent predictor of all outcomes. Regular involvement with a mentor was a consistent predictor of perceived academic and nonacademic benefit. Demographic characteristics were generally not predictive of outcomes. Peer mentoring is perceived by 1st-year medical students to provide psychosocial and academic benefits that appear to be independent of and complementary to those derived from faculty mentoring. These benefits can be established with minimal expenditure of institutional resources. Peer mentoring efficacy may be increased by more deliberate matching methods and by training mentors in provision of guidance and feedback, though further study is needed.