Background: Observationally, high plasma urate is associated with high risk of cancer. We used a Mendelian randomization design to test the hypothesis that high concentrations of plasma urate are associated with high cancer incidence and all-cause mortality observationally and genetically. Methods: We performed observational and genetic analyses using plasma urate and the urate solute carrier family 2 member 9 (SLC2A9) rs7442295 genotype in 86210 individuals from the Copenhagen General Population Study. Cancer and mortality end points were from national cancer and death registries. Incidences and risk of cancer and all-cause mortality were calculated using Cox regression, Fine and Gray competing-risks regression, and instrumental variable analyses. Results: During a median follow-up time of 3.9 years for cancer and 4.9 years for all-cause mortality, 3243 individuals received a diagnosis of cancer and 3978 died. Observationally, 50% higher plasma urate was associated with multivariable-adjusted hazard ratios of 1.11 (95% CI, 1.05-1.18) for cancer incidence and 1.07 (1.01-1.13) for all-cause mortality. Each A-allele of the SLC2A9 rs7442295 was associated with 9% higher plasma urate and hazard ratios of 1.07 (1.01-1.14) for cancer incidence and 1.07 (1.02-1.13) for all-cause mortality. In instrumental variable analyses, the odds ratios for a genetically determined 50% higher plasma urate was 1.22 (1.02-1.47) for cancer incidence and 1.49 (1.13-1.93) for all-cause mortality. Conclusions: High plasma urate was both observationally and genetically associated with high cancer incidence and high all-cause mortality, suggesting causal relationships.