Many doctoral students do not finish their studies. Social support has been shown to lower the likelihood of student withdrawal. I propose peer-mentoring as one of the practices that makes a significant difference in doctoral student attrition. In this longitudinal study, I investigated the influence of peer-mentoring on students' motivation and retention. Data were collected four times from 35 participants (a total of 140 surveys) in a Canadian university, where senior doctoral candidates mentored first year doctoral students. Data analysis indicates that students who received high levels of peer-mentoring experienced both steady and high autonomous motivation for their studies and low intention to leave. Implications for theory and practice are discussed.