Web 2.0 applications, such as social networking sites, are creating new challenges for medical professionalism. The scope of this problem in undergraduate medical education is not well-defined. To assess the experience of US medical schools with online posting of unprofessional content by students and existing medical school policies to address online posting. An anonymous electronic survey was sent to deans of student affairs, their representatives, or counterparts from each institution in the Association of American Medical Colleges. Data were collected in March and April 2009. Percentage of schools reporting incidents of students posting unprofessional content online, type of professionalism infraction, disciplinary actions taken, existence of institution policies, and plans for policy development. Sixty percent of US medical schools responded (78/130). Of these schools, 60% (47/78) reported incidents of students posting unprofessional online content. Violations of patient confidentiality were reported by 13% (6/46). Student use of profanity (52%; 22/42), frankly discriminatory language (48%; 19/40), depiction of intoxication (39%; 17/44), and sexually suggestive material (38%; 16/42) were commonly reported. Of 45 schools that reported an incident and responded to the question about disciplinary actions, 30 gave informal warning (67%) and 3 reported student dismissal (7%). Policies that cover student-posted online content were reported by 38% (28/73) of deans. Of schools without such policies, 11% (5/46) were actively developing new policies to cover online content. Deans reporting incidents were significantly more likely to report having such a policy (51% vs 18%; P = .006), believing these issues could be effectively addressed (91% vs 63%; P = .003), and having higher levels of concern (P = .02). Many responding schools had incidents of unprofessional student online postings, but they may not have adequate policy in place.