This study deals with the risk of injury to the bicyclist's head and the benefits of wearing a bicycle helmet in terms of reduction of injury severity or even injury avoidance. The accident data of 4,245 injured bicyclists as a randomized sample, collected by a scientific research team within the GIDAS project (German In-Depth Accident Study) were analyzed. Given that head injuries result in approximately 40% of bicycle-related crashes, helmet usage provides a sensible first-level approach for improving incidence and severity of head injuries. The effectiveness of the bicycle helmet was examined using descriptive and multivariate analysis for 433 bicyclists with a helmet and 3,812 bicyclists without a helmet. Skull fractures, severe brain injuries and skull base fractures were up to 80% less frequent for bicyclists wearing a helmet. Among individuals 40 years of age and older, a significant increase of severe head injuries occurred if no helmet was used compared to younger persons with helmet.