Background: Decision making for adjuvant chemotherapy is increasingly complicated for women with a new diagnosis of early stage breast cancer. Few decision tools are designed to help support informed systemic treatment decision-making, by improving knowledge and decision quality. Methods: We conducted a randomized controlled trial (RCT) of a tailored, comprehensive (locoregional and systemic treatment) and interactive decision tool (iCanDecide), compared with static online information. 537 newly diagnosed, early stage breast cancer patients were enrolled from 22 surgical practices. Participants were surveyed 5 weeks (N = 496; RR 92%) and those eligible for systemic treatment (N = 358) again at 9 months (N = 307; RR 88%). The main outcome for this analysis was knowledge about systemic treatment using 4 true/false items, categorized into high (3- 4 correct) vs. low (0-2 correct). We also assessed subjective decision quality (SDQ) for chemotherapy on a 5-point scale and dichotomized into high (4-5) vs. low (1-3) We evaluated the distribution of participants in each arm, and assessed the association between the study arm and the outcomes using bivariate and multivariable approaches. Results: Of the 358 respondents, 201 did not receive or intend to have chemotherapy. Significantly more intervention than control patients had high knowledge about systemic treatment (52.9% vs. 39.9%, p = 0.012). Overall SDQ for chemotherapy was slightly higher in intervention than control subjects (mean 4.8 vs. 4.6, p = 0.08). However, among women who did not receive chemotherapy, significantly more intervention subjects reported high SDQ than controls (87.1% vs. 75.2%, p = 0.06). Values significantly related to chemotherapy use included avoiding side effects, continuing to work, and being most extensive possible. Conclusions: We found that the interactive decision tool contributed significantly to higher knowledge about systemic treatment among eligible patients. We further found the tool shows promise for improving subjective decision quality, particularly in patients who choose not to have chemotherapy. Further work to integrate tools into the oncology clinical setting is needed. Clinical trial information: NCT01840163