Fatal injuries suffered by cyclists in vehicle-versus-cyclist accidents are investigated to provide information for the introduction of safety countermeasures. We analyzed characteristics of cyclist injuries in real fatal accidents and compared them with severity levels of head injury in impact tests against a road surface. In the accident analyses, we investigated the main body regions whose injuries led to fatalities using a macro vehicle-cyclist accident database of the Institute for Traffic Accident Research and Data Analysis of Japan. Using data from 2009 to 2013, we investigated the frequency of cyclist fatalities by gender, age group, vehicle speed, and the source of fatal head injury (impact with the vehicle or road surface). Results indicated that head injuries are the most common cause of cyclist fatalities in car-cyclist accidents. The results also indicated that the percentage of fatalities due to hip injuries was significantly higher for females than for males, and significantly higher for cyclists older than 65 years than those aged 13-59 years. It was clarified that fatalities due to head impacts against road surfaces were more common at low vehicle speeds among cyclists older than 60 years. The results of accident analyses also showed that the wearing of a helmet was extremely effective in preventing fatal cyclist accidents. In impact tests against a road surface, the impact was milder when the adult pedestrian headform impactor wore a helmet (head injury criterion of 860 versus 1157). It is also found that currently available helmets are designed for protection of a cyclist's head in the first impact only.