The J-START study examined the cumulative incidence of advanced breast cancer in women aged 40–49 years. This is a prespecified secondary analysis of the Japanese Strategic Anticancer Randomized Trial of Adjunctive Ultrasonography. Adjunctive ultrasonography reduced the cumulative incidence of advanced breast cancer in this age group. These findings highlight the value of integrating adjunctive ultrasonography into screening programs. The method is especially useful for women with dense breast tissue. The results are particularly relevant for the Asian population. The findings may influence future guidelines for breast cancer screening.