Many mobile phones allow two-way video communication, which permits callers to hear and see each other. If used during medical emergencies, bystanders can receive supervision and guidance from medical staff based on visual information. We investigated whether video calls from mobile phones could improve the confidence of lay rescuers. High school students (n = 180) were randomly assigned in groups of three to communicate via video calls or via ordinary mobile phone calls. They received realtime guidance from experienced nurse dispatchers at an emergency medical dispatch centre during 10-min scenarios of simulated cardiac arrest. Each student answered a questionnaire to assess understanding, confidence and usefulness of the technology. The mean age was 17.3 years in the video group and 17.9 years in the audio group. There were 27% male participants in the video group and 34% male participants in the audio group. Seventy-three percent of the students in the video group and 71% in the audio group reported previous cardiopulmonary resuscitation training. Rescuers who had not used video phones had a greater tendency to comment on immature video call technology, while some who had used video phones complained about poor sound quality during video calls. The majority of rescuers in both groups believed that video calls were superior to audio calls during medical emergencies, and this proportion was significantly higher in the video group (P = 0.0002). We found that visual contact and supervision through video calls improved rescuers' confidence in stressful emergencies.