<p>This study used data from the Utsunomiya COVID-19 seROprevalence Neighborhood Association (U-CORONA) study in Utsunomiya, Japan, to examine the association between positive childhood experiences, flourishing and biomarkers adjusting for adverse childhood experiences. The survey was conducted in June-July (wave 1) and October (wave 2) 2020 under the COVID-19 pandemic among randomly selected community residents. This study assessed seven positive childhood experiences and thirteen adverse childhood experiences, including child maltreatment, household dysfunction, and environmental adversities, within the Japanese context. The analytic sample was participants aged 18 and over who completed both surveys and collected blood samples (<i>n</i> = 397; mean age 53.4 years; 47.1% male). In multiple linear regression analyses, having a greater number of positive childhood experiences was associated with a higher flourishing index (i.e., comprehensive state of well-being), although having adverse childhood experiences was not significantly associated. In addition, a higher number of positive childhood experiences was associated with a favorable inflammation state, as measured by lower White Blood Cell (WBC) counts and C-Reactive Protein (CRP) levels, even after adjusting for adverse childhood experiences. Positive childhood experiences were associated with flourishing and inflammatory biomarkers in adulthood, even after adjusting for adverse childhood experiences. These findings suggest that positive relational resources in childhood may be associated with lifelong well-being, potentially through biological pathways, offering insights for future public health strategies focused on early-life positive experiences.</p>