The objective of this study was to evaluate the clinical utility of a decisional aid for mental capacity assessments which was developed using a group judgment methodology. This was carried out by a randomized, controlled trial. The subjects comprised 64 University of Toronto psychiatry residents in postgraduate years 1 through 4. Residents were randomized to carry out mental capacity assessments on simulated cases with, or without, the use of the decisional aid. The main outcome measure was the extent of agreement between the mental capacity determinations of residents and those of experts. There was no difference between the intervention and control groups with respect to the overall mean level of agreement with experts (0.87 vs 0.86, p = 0.88; 95% confidence interval for the difference between the study groups, -0.07 to +0.08). A logistic regression analysis, which adjusted for imbalances between the groups, also revealed no difference between the groups in their agreement with experts. The mean time per competency assessment was significantly longer in the intervention group (19.1 vs 10.8 min; p < 0.001). It was concluded that the decisional aid did not improve the ability of the psychiatry residents to make mental capacity assessments on simulated cases. Despite relatively limited formal training, the psychiatry residents had a high level of agreement with experts.