A decision aid for the surgical treatment of early breast cancer was evaluated in a randomized controlled trial. The decision aid, a tape and workbook, includes explicit presentation of probabilities, photographs and graphics, and a values clarification exercise. Community surgeons were randomized to use the decision aid or a control pamphlet. Patients completed a questionnaire prior to using the decision aid, after reviewing it but prior to surgery, and 6 months after enrollment. There was no difference in anxiety, knowledge, or decisional regret across the 2 groups. There was a nonsignificant trend toward lower decisional conflict in the decision aid group. A subgroup of women who were initially leaning toward mastectomy or were unsure had lower decisional conflict. Although the decision aid had minimal impact on the main study outcomes, a subgroup may have benefited. Such subgroups should be identified, and appropriate decision support interventions should be developed and evaluated.