Recent studies suggested that higher BMI and serum urate levels were associated with lower risk of developing dementia. However, these reverse relationships remain controversial, and whether serum urate and BMI confound each other is not well established.To investigate the independent associations of BMI and urate, as well as their interaction with the risk of developing dementia.We analyzed a cohort of 502,528 individuals derived from the UK Biobank that included people aged 37 to 73 years for whom BMI and urate were recorded between 2006 and 2010. Dementia was ascertained at follow-up using linkage to electronic health records.During a median of 8.1 years of follow-up, a total of 2138 participants developed dementia. People who were underweight had an increased risk of dementia (hazard ratio [HR] = 1.91, 95% confidence interval [CI]: 1.24-2.97) compared with people of healthy weight. However, the risk of dementia continued to fall as weight increased, as those who were overweight and obese were 19% (HR = 0.81, 95%: 0.73-0.90) and 22% (HR = 0.78, 95% CI: 0.68-0.88) less likely to develop dementia than people of healthy weight. People in the highest quintile of urate were also associated with a 25% (HR = 0.75, 95% CI: 0.64-0.87) reduction in the risk of developing dementia compared with those who were in the lowest quintile. There was a significant multiplicative interaction between BMI and urate in relation to dementia (P for interaction = 0.004), obesity strengthens the protective effect of serum urate on risk of dementia.Both BMI and urate are independent predictors of dementia, and there are inverse monotonic and dose-response associations of BMI and urate with dementia.