Introduction: The controversy surrounding commercial support for continuing medical education (CME) programs has led to policy changes, but data show no significant difference in perceived bias between commercial and noncommercial CME. Indeed, what attendees perceive as commercial influence is not fully understood. We sought to clarify what "sources" contribute to attendees' perceptions of commercial influence in non-industry-supported CME programs, and how attendees perceive that this influence manifests itself on both speaker and program levels. Methods: Evaluation forms were received from 1 544 attendees at 14 live noncommercially supported CME programs in 2006, 2007, and 2010. Attendees rated perceived commercial influence for each lecture and the entire program. Using open-ended and "check all that apply" questions, participants specified perceived "sources" and "manifestations" of commercial influence. Results: Attendees rating individual lectures but not the entire program as commercially influenced accounted for 59.9% of those who identified bias. The most frequently endorsed source of commercial influence was individual speakers' funding, and the most listed manifestations were speakers' mentions of pharmaceuticals and expressions of personal opinions. Rating the entire program commercially influenced correlated with whether attendees considered the funding of referenced research a source of influence. Discussion: CME attendees consider a broad spectrum of factors when reporting commercial influence. Evaluation forms should include bias questions per lecture as well as items to clarify perceived sources and manifestations of commercial influence. (Contains 3 tables and 1 figure.)