BACKGROUND. Despite the potential benefits of genetic testing for hereditary nonpolyposis colorectal cancer (HNPCC) risk, individuals can find the genetic testing decision-making process complicated and challenging. The goal of the current study was to measure the effectiveness of a tailored decision aid designed specifically to assist individuals to make informed decisions regarding genetic testing for HNPCC risk. METHODS. In all, 153 individuals were randomized to receive the decision aid or a control pamphlet at the end of their first genetic counseling consultation. Of these, 109 (71.2%) completed the first questionnaire 1 week after consultation, whereas 95 (62.1%) completed the 6-month follow-up questionnaire. RESULTS. Although the decision aid had no significant effect on postdecisional regret or actual genetic testing decision, the trial results demonstrated that participants who received the decision aid had significantly lower levels of decisional conflict (ie, uncertainty) regarding genetic testing (chi-square(1) = 8.97; P = .003) and were more likely to be classified as having made an informed choice concerning genetic testing (chi-square(1) = 4.37; P = .037) than participants who received a control pamphlet. Also, men who received the decision aid had significantly higher knowledge levels regarding genetic testing compared with men who received the control pamphlet, whereas no such differences were found for women (chi-square(2) = 6.76; P = .034). CONCLUSIONS. A decision aid for individuals considering genetic testing for HNPCC is an effective intervention to reduce uncertainty and assist individuals to make an informed choice regarding genetic testing for HNPCC after genetic counseling.