To examine trends and demographic characteristics of syphilis incidence in Japan using a large nationwide claims database with family linkage, with particular focus on differences by sex, age, HIV status and family relationships.

Retrospective cohort study.

JMDC claims database (JMDC Inc, Tokyo, Japan), a nationwide administrative claims database in Japan, using data from 2016 to 2023.

Individuals aged 16–59 years enrolled in the JMDC database, including employees of medium-to-large companies and their dependents (n=12.5 million).

Syphilis cases were defined by International Classification of Diseases, 10th Revision (ICD-10) codes (A50–A53) with concurrent treatment with relevant antibiotics. We determined syphilis incidence rates per 100 000 person-years, stratified by sex, age, HIV status and family relationships. We also investigated within-couple concordance patterns and reinfection rates.

Among 16.4 million individuals, 9357 syphilis infections were identified among 8881 individuals. Incidence increased markedly during the pandemic, reaching 48.2 (men) and 12.9 (women) per 100 000 person-years in 2023. Men showed consistently high incidence in their 20s–50s, whereas female incidence peaked in the 10s–20s. Among 2 294 184 married couples, dependent women (ie, housewives) showed comparably high incidence to age-matched men (10–20 per 100 000 person-years). In 1286 couples with at least one syphilis case, 12.4% of wives in their 20s were also diagnosed, compared with 2%–3% in older groups. In 20s couples, the proportion of syphilis among wives only and husbands only was similar. Subgroup analysis revealed notably high incidence among unmarried female dependent youths (2022: 66.7 per 100 000 person-years). Individuals living with HIV had substantially elevated incidence (3000–15 000 per 100 000 person-years) and reinfection rates.

Using a large claims database with family linkage, we found that while male syphilis incidence remained dominant, high rates were also observed among dependent women and youths. These findings suggest that syphilis risk may extend beyond traditionally recognised high-risk populations and emphasise the need for targeted screening and preventive strategies in broader demographic groups.