Context:The assignment of direction and causality within networks of observational associations is problematic outside randomized control trials, and the presence of a causal relationship between body mass index (BMI) and C-reactive protein (CRP) is disputed.Objective:Using reciprocal Mendelian randomization, we aim to assess the direction of causality in relationships between BMI and CRP and to demonstrate this as a promising analytical technique.Participants and methods:The study was based on a large, cross-sectional European study from Copenhagen, Denmark. Genetic associates of BMI (FTO(rs9939609)) and circulating CRP (CRP(rs3091244)) have been used to reexamine observational associations between them.Results:Observational analyses showed a strong, positive association between circulating CRP and BMI (change in BMI for a doubling in logCRP of 1.03 kg m(-2) (95% confidence interval (95% CI): 1.00, 1.07), P