To evaluate helmet use in California before and after the introduction of an unrestricted helmet use law on January 1, 1992, observations of motorcycles and their riders were made at 60 locations in seven California counties, twice before and four times after the law was introduced. Helmet use increased from about 50% in 1991 to more than 99% throughout 1992. Compliance was achieved despite variations in helmet use by motorcycle design and road type. Seven percent of riders used nonstandard helmets after the law. With adequate enforcement, unrestricted helmet use laws can achieve almost 100% compliance and reduce the number of people riding motorcycles.