After a decade of promotion and education, legislation for mandatory helmet wearing by bicyclists in Victoria was introduced on 1 July 1990. The legislation was a world first. Comparison of 1710 bicyclist casualties wearing and not wearing helmets has demonstrated that wearing helmets certified to the Australian Standard reduces the head injury risk by at least 39% and lessens head injury severity. Simulated impact testing of helmets has shown that they provide protection in most impacts including collisions involving a motor vehicle. Legislation for mandatory helmet wearing in Victoria has led to increased wearing rates and marked reductions in bicyclist fatalities and head injuries. The Victorian experience gives substantial support to the introduction of legislation for mandatory helmet wearing by bicyclists.