Relatively little is known about how, from whom, and under what conditions residents say they most effectively learn. We examined the relationships between residents' self-reported ratings of 11 different sources of learning and a number of empirical variables, using a national, random sample of postgraduate year (PGY) 1 and PGY-2 residents in the 1998-1999 training year. Residents were surveyed by mail. Completed surveys were received from 64.2% of 5616 residents contacted. The most often reported sources of learning were other residents and attending physicians. Ratings varied by specialty, level of training, and US medical graduates (USMGs) versus international medical graduates (IMGs). Factor analysis identified 3 primary modes of learning: faculty-organized, peer-oriented, and self-directed. Residents in different specialties varied in their use of these 3 sources of learning. IMG residents reported significantly less learning from peers and more self-directed learning. Increased resident duty hours were associated with a decrease in faculty-organized and self-directed learning, and an increase in peer-oriented learning.