In a prescient move more than a decade ago, the Medical Research Council and Wellcome Trust decided to establish the large UK Biobank prospective cohort to support the investigation of risk factors for the major diseases of middle and old age. 1, 2 Recruitment of more than 500 000 men and women aged 40–69 years was successfully achieved during 2006–10 and their health is being followed long term. On March 30, 2012, the UK Biobank resource is launched for use by all researchers—without exclusive or preferential access—for any healthrelated research that is in the public interest. The challenges of understanding the determinants of common life-threatening and disabling diseases are substantial. Such conditions are typically caused by many different exposures that might each have moderate effects and interact with each other in complex ways. 3, 4 To investigate a wide range of exposures, extensive information needs to be collected through questionnaires and physical measurements, as well as by storing biological samples that allow many different types of assay (eg, genetic, proteomic, metabonomic, or biochemical). Moreover, to study reliably the sort of effects of different exposures that it is plausible to expect, many thousands of cases of a specific disease may be required. 4

Prospective cohorts have a number of advantages for the comprehensive and reliable quantification of the combined effects of lifestyle, environment, genes, and other exposures on health outcomes. 3, 5 In particular, exposures can be assessed before they are affected by disease or its treatment, or by a person’s response to developing disease. Diseases can also be studied that are not readily investigated by retrospective studies (eg, dementia or rapidly fatal conditions). Moreover, the overall beneficial and adverse effects of a specific exposure on the life-time risks of multiple health outcomes can be considered (eg, associations of obesity with different causes of death6). Prospective studies do, however, need to be large because only a small proportion of the participants will develop any particular disease.