OBJECTIVE: To evaluate the potential long-term health impact of helmet law, we calculated the loss of quality-adjusted life year (QALY) under different proportions of helmet wearing among the motorcyclists. METHODS: The quality-adjusted life expectancies for helmeted and non-helmeted motorcyclists were estimated by adjusting the survival function based upon the Head Injury Registry with quality of life measures assessed under the EQ-5D questionnaire. We took Hualien County, where a lower rate of helmet wearing (77%) was reported, as an example to calculate the expected numbers of prevented head-injured cases by multiplying the population at risk with the incidence rate of head injury for helmeted and non-helmeted motorcyclists. As different proportions of helmet-wearing and different proportions of full face helmet in motorcyclists were assumed, the expected numbers of prevented cases were calculated, which were multiplied with the loss of QALE of an average case to predict the potential benefit of helmet use. RESULTS: Under the current proportion of helmet wearing, the annual loss associated with head injured was 4346.9 QALYs in Hualien County. If the proportion of helmet wearing could be increased to 100%, the health benefit saved was estimated 1434.3 QALYs. If 80% of them used full face helmet, the total gain was increased to 2500 QALYs. CONCLUSION: The health benefit of helmet protection for head injury can be determined under units of quality-adjusted life year (QALY) and directly applied in future cost-effective analyses for public health policy.