We describe the user-centered development of an electronic medical record–based portal, “MyAsthma,” designed to facilitate shared decision making in pediatric asthma. Interviews and focus groups with 7 parents of children with asthma and 51 clinical team members elicited 2 overarching requirements: that the portal should support sustained communication and ensure patient safety. Parents and clinicians prioritized features including collecting parent and child concerns and goals; symptom, side effect, and medication adherence tracking with decision support; and accessible educational materials. Iterative usability testing refined the system. MyAsthma provides a model for using technology to foster shared decision making in ambulatory care settings.