This study examined whether: (1) brief training in motivational interviewing (MI) can prepare mental health nurses (MHNs) to provide MI to patients; and (2) this MI impacts on patients with respect to premature discharge. Six MHNs on an inpatient eating disorder unit were trained in MI, and their treatment adherence and competence were evaluated at post-training and 2-month follow-up. Premature discharge was examined by comparing a 3-month period in 2009 before MI administration with 2010 when MI was being administered. MHNs significantly improved their MI adherence and competence. Satisfaction with the training was high as was patient satisfaction with MI. Premature discharge rates significantly decreased. Brief training in MI is sufficient to significantly increase competency and adherence in the practice of MI by MHNs, which may in turn be effective in improving patients' treatment adherence by reducing premature discharge rates. Future research will need to utilize a randomized controlled design in order to further investigate these findings.