PURPOSE: Bicycle riding is a healthy and cheap form of transportation and a popular form of recreation. This study aims to evaluate the epidemiology and the factors affecting the severity of bicycle-related injuries and to find effective methods for preventing injuries. METHODS: This is a retrospective study of the patients' characteristics, the injury severity, the injury location, the effect of safety equipment on the bicycle injury based on information collected through the NEDIS (National Emergency Department Information System) from 1,284,429 patients who visited 55 emergency rooms nationwide. RESULTS: During the research period throughout 55 emergency rooms 5,671 patients were seen because of bicycle injuries. The male-to-female ratio was 3.1:1, the median age 28 years old, and 42.6% of the patients were younger than 20 years old. Injury sites were mostly in the extremities(46.7%), the head(32.4%), and the face(14%), and 70.3% of severely injured patients had accompanying head trauma. Males aged 65 and older was possibly associated with severe head trauma. Of the patients who provided helmet information 4.8%(71 patients) used helmets, and 95.2%(1392 patients) did not. In this group of patients providing helmet information, none of those wearing helmets died, but 0.4% of those not wearing helmets died. CONCLUSION: In this study, most bicycle injuries occurred in young adult men. From now, there seems to be a need for more effort on publicity activities on bicycle injuries and on the education of children and teenagers, who show a high incidence rate, and of senior citizens (over 65 years old) who show a high severity rate, about using a safety helmet to reduce the severity of injury.