We investigate the effects of guided group work sessions on graduate student performance on a quantum mechanics assessment. Data from a single large Midwestern university were taken over a five-year period, during which guided group work sessions were offered to accompany the graduate-level quantum mechanics course. Students were pre- and post-tested using a set of mostly conceptual items that we call the graduate quantum mechanics assessment. The reliability and validity of this assessment are addressed. A mixed linear model is used to analyze the dependence of post-test scores on factors such as group work attendance, pretest scores, GRE Physics scores, and others. We find a statistically significant effect of group work attendance on post-pre gains, specifically that attendance of one 60-min group work session improves performance on a related post-test item by 6.4%, administered 2–10 weeks after the session. We discuss the lack of a randomized control group and address possible confounding effects such as student self-selection, and attitudinal and motivational factors. Overall, the results of this study indicate that guided group work sessions at the graduate level can be feasible and effective. We note preliminary observations of differences in group interactions and classroom logistics compared to group work at the undergraduate level.