Managing personal aspects of health is challenging for many patients, particularly those facing a serious condition such as cancer. Finding experienced patients, who can share their knowledge from managing a similar health situation, is of tremendous value. Users of health-related social software form a large base of such knowledge, yet these tools often lack features needed to locate peers with expertise. Informed directly by our field work with breast cancer patients, we designed a patient expertise locator for users of online health communities. Using feedback from two focus groups with breast cancer survivors, we took our design through two iterations. Focus groups concluded that expertise locating features proved useful for extending social software. They guided design enhancements by suggesting granular user control through (1) multiple mechanisms to identify expertise, (2) detailed user profiles to select expertise, and (3) varied collaboration levels. Our user-centered approach links field work to design through close collaboration with patients. By illustrating trade-offs made when sharing sensitive health information, our findings inform the incorporation of expertise locating features into social software for patients.