Background Inflammation plays a central role in cardiometabolic disease and may represent a mechanism linking low socioeconomic status (SES) in early life and adverse cardiometabolic health outcomes in later life. Accumulating evidence suggests an association between childhood SES and adult inflammation, but findings have been inconsistent. Methods We conducted a systematic review and metaanalysis of observational studies to quantify the association between childhood (age < 18 years) SES and the inflammatory marker C reactive protein (CRP) in adulthood. Studies were identified in Medline and Embase databases, and by reviewing the bibliographies of articles published from 1946 to December 2015. Study-specific estimates were combined into metaanalyses using random-effects models. Results 15 of 21 eligible studies (n=43 629) were ultimately included in two separate meta-analyses. Compared with those from the most advantaged families, participants from the least advantaged families had 25% higher CRP levels (ratio change in geometric mean CRP: 1.25; 95% CI 1.19 to 1.32) in minimally adjusted analyses. This finding was attenuated by the inclusion of adult body mass index (BMI) in adjusted models, suggesting BMI has a strong mediating role in CRP levels. Conclusions We observed an inverse association between childhood SES and adulthood CRP, potentially mediated through BMI. Investigating how childhood SES is associated with childhood BMI and CRP would provide insight into the effective timing of social and clinical interventions to prevent cardiometabolic disease.