Background: Despite reported benefits, many women do not attend breast cancer support groups. Abundant online resources for support exist, but information regarding the effectiveness of participation is lacking. We report the results of a Twitter breast cancer support community participant survey. Objective: The aim was to determine the effectiveness of social media as a tool for breast cancer patient education and decreasing anxiety. Methods: The Breast Cancer Social Media Twitter support community (#BCSM) began in July 2011. Institutional review board approval with a waiver of informed consent was obtained for a deidentified survey that was posted for 2 weeks on Twitter and on the #BCSM blog and Facebook page. Results: There were 206 respondents to the survey. In all, 92.7% (191/206) were female. Respondents reported increased knowledge about breast cancer in the following domains: overall knowledge (80.9%, 153/189), survivorship (85.7%, 162/189), metastatic breast cancer (79.4%, 150/189), cancer types and biology (70.9%, 134/189), clinical trials and research (66.1%, 125/189), treatment options (55.6%, 105/189), breast imaging (56.6%, 107/189), genetic testing and risk assessment (53.9%, 102/189), and radiotherapy (43.4%, 82/189). Participation led 31.2% (59/189) to seek a second opinion or bring additional information to the attention of their treatment team and 71.9% (136/189) reported plans to increase their outreach and advocacy efforts as a result of participation. Levels of reported anxiety before and after participation were analyzed: 29 of 43 (67%) patients who initially reported “high or extreme” anxiety reported “low or no” anxiety after participation (P<.001). Also, no patients initially reporting low or no anxiety before participation reported an increase to high or extreme anxiety after participation. Conclusions: This study demonstrates that breast cancer patients’ perceived knowledge increases and their anxiety decreases by participation in a Twitter social media support group.