Background This study was conducted to examine whether passage of the mandatory motorcycle helmet law in 1997 reduced the regional inequality in mortality from road traffic injuries (RTIs) across 22 cities/counties in Taiwan.

Methods We calculated the absolute (between-group variance, BGV) and relative (rate ratio between the city/county with the highest and lowest rate, RR) terms of inequality for the overall and motorcycle-related RTI mortality rates, the rate of helmet use and three other explanatory factors associated with RTI mortality at the city/county level from 1997 through 2008.

Results The BGV of the overall and motorcycle-related RTI mortality rates across the 22 cities/counties showed persistently decreasing trends from 1997 to 2008; however, the RR of RTI mortality first increased and then levelled off from 2002. The decreasing trend in inequality was most prominent in males aged 0–24 years. The BGV and RR of the rate of motorcycle helmet use decreased after passage of the law but increased from 2002 onwards.

Conclusion In Taiwan, passage of the mandatory motorcycle helmet law reduced the regional inequality in RTI mortality; however, a resurgence in regional inequality in the helmet use rate  years after passage of the helmet law was noted. It is therefore necessary to monitor the helmet use rate after passage of such a law to ensure the effect of a reduction in regional inequality in RTI mortality.