OBJECTIVE: To estimate the impact of a mandatory motorcycle helmet law in Taiwan. METHODS: Taiwan passed a mandatory helmet law in June 1997. Data were collected retrospectively from police reports, which include hospital data, to compare six months pre-law June to November 1996) with the same six months post-law (June to November 1997). RESULTS: Motorcycle fatalities decreased 14% after the introduction of the helmet law. Head injury fatalities fell 22% while fatalities from injuries to other bodily areas rose 20%. Non-fatal motorcycle injuries fell 31%. Non-fatal head injuries fell 44%; non-fatal injuries to other body parts fell 23%. CONCLUSION: This study indicates that large, immediate public health benefits resulted from the mandatory motorcycle helmet law in Taiwan.