Abstract Background The UK, like other developed countries, is experiencing a marked change in the age structure of its population characterised by increasing life expectancy and continuing growth in the older fraction of the population. There is remarkably little up-to-date information about the health of the oldest old (over 85 years), demographically the fastest growing section of the population. There is a need, from both a policy and scientific perspective, to describe in detail the health status of this population and the factors that influence individual health trajectories. For a very large proportion of medical conditions, age is the single largest risk factor. Gaining new knowledge about why aged cells and tissues are more vulnerable to pathology is likely to catalyse radical new insights and opportunities to intervene. The aims of the Newcastle 85+ Study are to expose the spectrum of health within an inception cohort of 800 85 year-olds; to examine health trajectories and outcomes as the cohort ages and their associations with underlying biological, medical and social factors; and to advance understanding of the biological nature of ageing. Methods A cohort of 800 85 year olds from Newcastle and North Tyneside will be recruited at baseline and followed until the last participant has died. Eligible individuals will be all those who turn 85 during the year 2006 (i.e. born in 1921) and who are registered with a Newcastle or North Tyneside general practice. Participants will be visited in their current residence (own home or institution) by a research nurse at baseline, 18 months and 36 months. The assessment protocol entails a detailed multi-dimensional health assessment together with review of general practice medical records. Participants will be flagged with the NHS Central Register to provide details of the date and cause of death. Discussion The Newcastle 85+ Study will address key questions about health and health-maintenance in the 85+ population, with a particular focus on quantitative assessment of factors underlying variability in health, and on the relationships between health, nutrition and biological markers of the fundamental processes of ageing.