Although the traumatic injury death rate in the United States decreased during the last 20 years, the percent of all injury deaths attributable to brain injuries has remained steady. Head injuries are a leading cause of injury among motorcycle riders in crashes, and the helmet is an effective measure to reduce these injuries. To reduce the burden and cost of motorcycle injuries, many states have increased helmet use by introducing mandatory helmet legislation. This report presents evidence on the effects of the motorcycle helmet and helmet use legislation--two interrelated prevention strategies which decrease traumatic brain injury among motorcycle riders. Nonfatally injured motorcyclists were included if they crashed between January 1, 1991 and December 31, 1993 and were treated in one of 18 hospitals in 10 California counties. Medical records for all injured motorcyclists admitted to these 18 hospitals and those treated in the emergency department and released in eight of these hospitals were individually reviewed. Complete lists of injury diagnoses for each rider were coded according to the 1990 Abbreviated Injury Severity Scale by trained clinical staff. Before the universal helmet law was introduced (when only a third of injured riders wore helmets) 38.2% of riders sustained head injuries. When helmet use increased to over 85% of injured riders following the law, less than 25% of riders sustained head injuries. Both the severity and the number of head injuries per individual rider decreased after the mandatory helmet use law led to increased helmet use. Riders wearing helmets suffered fewer skull fractures, fewer intracranial injuries, had less frequent and shorter periods of loss of consciousness, more favorable GCS scores, and shorter hospital stays. Mandatory motorcycle helmet use laws are an effective mechanism to increase helmet use among riders and thus prevent head and brain injuries resulting from motorcycle crashes.