Effective treatment of chronic illness requires active engagement of patients in managing their own conditions. This research explores the influence of provider support for self-management on patients' performance of self-care activities, a widely presumed but under-studied relationship, particularly among underserved populations. We surveyed an ethnically diverse, primarily low-income sample of 956 patients (or parents of pediatric patients) with diabetes or asthma in 17 outpatient teaching settings across the country. Multivariate analysis established that patients with strongly positive assessments of their providers were substantially more confident in self-care. Further, among patients with diabetes, high assessments of provider support, in comparison with low, were significantly associated with performance of self-management tasks, amounting to approximately one more day per week. These relationships were significant for patients with either illness and from varied socio-economic backgrounds. The results provide evidence of the validity of our measure of provider support, its relevance to underserved populations, and its usefulness for evaluating quality of care.