On July 1, 1990, a law requiring wearing of an approved safety helmet by all bicyclists (unless exempted) came into effect in Victoria, Australia. Some of the more important steps that paved the way for this important initiative (believed to be the first statewide legislation of its type in the world) are described, and the initiative's effects are analysed. There was an immediate increase in average helmet-wearing rates from 31% in March 1990 to 75% in March 1991, although teenagers continued to show lower rates than younger children and adults. The number of insurance claims from bicyclists killed or admitted to hospital after sustaining a head injury decreased by 48% and 70% in the first and second years after the law, respectively. Analysis of the injury data also showed a 23% and 28% reduction in the number of bicyclists killed or admitted to hospital who did not sustain head injuries in the first and second post-law years, respectively. For Melbourne, where regular annual surveys of helmet wearing have been conducted, it was possible to fit a logistic regression model that related the reduction in head injuries to increased helmet wearing. Surveys in Melbourne also indicated a 36% reduction in bicycle use by children during the first year of the law and an estimated increase in adult use of 44%.