BACKGROUND: The motivation to volunteer on a medical service trip (MST) may involve more than a simple desire for philanthropy. Some volunteers may be motivated by an intrinsic interest in volunteering in which the context of the volunteer activity is less important. Others may volunteer because the context of their volunteering is more important than their intrinsic interest in volunteering. Furthermore, MSTs may pose a variety of ethical problems that volunteers should consider prior to engaging in a trip. This study evaluated the motivations and barriers for graduate health care students volunteering for an MST to either the Dominican Republic or Mississippi. Volunteers' understanding of some of the ethical issues associated with MSTs was also assessed. METHODS: Thirty-five graduate health professions students who volunteered on an MST were asked to complete an online survey. Students' motivations and barriers for volunteering were assessed using a 5-point Likert scale and Fisher's exact test. Ethical understanding of issues in volunteering was assessed using thematic analysis. RESULTS: Students' motivations for volunteering appeared to be related to the medical context of their service more than an inherent desire for volunteer work. Significant differences were seen in motivations and barriers for some student groups, especially those whose volunteer work had less opportunity for clinical service. Thematic analysis revealed two major themes and suggested that students had an empirical understanding that volunteer work could have both positive and negative effects. CONCLUSIONS: An understanding of students' motivations for volunteering on an MST may allow faculty to design trips with activities that effectively address student motivations. Although students had a basic understanding of some of the ethical issues involved, they had not considered the impact of a service group on the in-country partners they work with.