Classroom discussion is perhaps the most frequently used "active learning" strategy. However, instructors are often concerned about students who are less inclined to participate voluntarily. They worry that students not involved in the discussion might have lower quality learning experiences. Although instructors might consider whether to call on a student whose hand is not raised ("cold-call"), some instructors resist cold-calling fearing that the student will feel uncomfortable. This study examines the impact of cold-calling on students' voluntary participation in class discussions and their comfort participating in discussions. The results demonstrate that significantly more students answer questions voluntarily in classes with high cold-calling, and that the number of students voluntarily answering questions in high cold-calling classes increases over time. Furthermore, students in classes with high cold-calling answer more voluntary questions than those in classes with low cold-calling; this also increases over time. Finally, in classes with high cold-calling, students' comfort participating in class discussions increases while in classes with low cold-calling, students' comfort participating does not change. Research findings show that cold-calling can be done fairly extensively without making students uncomfortable. Thus, the research reported here provides support for using this instructional strategy to engage more students to participate in discussions.