BackgroundThe 2015 Japan Standard Population (JSP) was established in response to changes in the age structure. However, the effects of major updates, especially the recategorization of older age groups, for interpreting various health metrics have not been clarified.MethodsPopulation data were collected and estimated for older age categories (85–89, 90–94, and ≥95 years). Data on the number of deaths were also collected from the Vital Statistics. We recalculated the all-cause and leading cause-specific age-standardized mortality rate (ASMR) using the 2015 JSP by the direct standardization method for data from 1950 to 2020. We compared ASMRs calculated using the 2015 JSP with those calculated using the 1985 JSP. Pearson’s correlation coefficients were used to evaluate the consistency of mortality trends between the 2015 and 1985 JSPs.ResultsThe absolute all-cause ASMRs calculated using the 2015 JSP were 2.22–3.00 times higher than those calculated using the 1985 JSP. The ASMR ratios increased gradually over time. While trends in all-cause and cause-specific ASMRs calculated using the 2015 JSP and 1985 JSP were generally highly correlated (Pearson’s correlation coefficient [r] = 0.993 for all-cause), correlations were relatively low for malignant neoplasms (r= 0.720 for men andr= 0.581 for women) and pneumonia/bronchitis (r= 0.543 for men andr= 0.559 for women) due to non-monotonous trends over time and fluctuations in earlier time periods.ConclusionThe effect of introducing the new JSP for interpreting trends in all-cause mortality was considered minimal. However, caution is needed when interpreting trends in some cause-specific mortality rates.