Motivational interviewing (MI) is a brief client-centred intervention that increases intrinsic motivation for change. Little research has been conducted on MI to promote academic behaviours, but two studies found that one session of MI did improve middle-school students' math grades [Strait, G., Smith, B., McQuillin, S., Terry, J., Swan, S., & Malone, P. (2012). "A randomized trial of motivational interviewing to improve middle school students' academic performance." "Journal of Community Psychology," 40, 1032-1039; Terry, J., Strait, G., Smith, B., & McQuillin, S. (2013). "Motivational interviewing to improve middle school students' academic performance: A replication study." "Journal of Community Psychology," 41, 902-909]. This study examines dosage effects of MI by randomly assigning 42 middle-school students to either one or two sessions of MI (n = 21 per group). Significant dosage effects were found for math, science and history grades, with effect sizes of 0.55, 0.58 and 0.47, respectively. Considering that math effects were 0.47 and 0.37 in the prior studies, and math was the only significant change produced by one session of MI, these results strongly suggest that two sessions of MI are superior to one. Future studies should replicate these findings and explore if even higher doses produce stronger effects.