<title>Abstract</title>
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<title>Background</title>
The complex role of urbanisation in heat-mortality risk has not been fully studied. Japan has experienced a rapid population increase and densification in metropolitan areas since the 2000s; we investigated the effects of population concentration in metropolitan areas on heat-mortality risk using nationwide data.


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<title>Methods</title>
We collected time-series data for mortality and weather variables for all 47 prefectures in Japan (1980–2015). The prefectures were classified into three sub-areas based on population size: lowest (&amp;lt;1 500 000), intermediate (1 500 000 to 3 000 000), and highest (&amp;gt;3 000 000; i.e. metropolitan areas). Regional indicators associated with the population concentration of metropolitan areas were obtained.


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<title>Results</title>
Since the 2000s, the population concentration intensified in the metropolitan areas, with the highest heat-mortality risk in prefectures with the highest population. Higher population density and apartment % as well as lower forest area and medical services were associated with higher heat-mortality risk; these associations have generally become stronger since the 2000s.


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<title>Conclusions</title>
Population concentration in metropolitan areas intensified interregional disparities in demography, living environments, and medical services in Japan; these disparities were associated with higher heat-mortality risk. Our results can contribute to policies to reduce vulnerability to high temperatures.


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