The decision aid called ''Adjuvant Online'' (Adjuvant! for short) helps breast cancer patients make treatment decisions by providing numerical estimates of treatment efficacy (e.g., 10-y relapse or survival). Studies exploring how patients' numeracy interacts with the estimates provided by Adjuvant! are lacking. Pooling across 2 studies totaling 105 women with estrogen receptor-positive, early-stage breast cancer, the authors explored patients' treatment expectations, perceived benefit from treatments, and confidence of personal benefit from treatments. Patients who were more numerate were more likely to provide estimates of cancer-free survival that matched the estimates provided by Adjuvant! for each treatment option compared with patients with lower numeracy (odds ratios of 1.6 to 2.4). As estimates of treatment efficacy provided by Adjuvant! increased, so did patients' estimates of cancer-free survival (0.37 > r(s) > 0.48) and their perceptions of treatment benefit from hormonal therapy (r(s) = 0.28) and combined therapy (r(s) = 0.27). These relationships were significantly more pronounced for those with higher numeracy, especially for perceived benefit of combined therapy. Results suggest that numeracy influences a patient's ability to interpret numerical estimates of treatment efficacy from decision aids such as Adjuvant!.