Dramatic changes in global HIV and AIDS estimates were publicised at the end of 2007,1 with a notable downward adjustment in the estimated number of people living with HIV (PLHIV) from 39.5 million (range 34.1–47.1 million) published in 20062 to 33.2 million (range 30.6–36.1 million).1 Both the original and revised estimates are by any measure catastrophic; however, the new lower estimates come at a time when global disease burden estimates are under intense scrutiny.3 4 While the 2007 AIDS Epidemic Update report noted that the downward adjustments were the result of better data leading to changes in assumptions and thereby estimates, we recognise the need for the highest level of transparency and opportunity for scientific critique. Since 2004 we have provided detailed descriptions of the tools and assumptions used in generating HIV and AIDS estimates, as well as the data and analyses underpinning these assumptions.5^–7 This current supplement assembles important new data relating to several assumptions used for the new HIV and AIDS estimates. By bringing together a new collection of methodological papers in this supplement, we aim to provide easy access to the scientific basis underlying the latest HIV and AIDS estimates for 2007.1 8 9

The process of synthesising new data, reviewing assumptions and providing guidance on methods is driven by the UNAIDS Reference Group on Estimates, Modelling and Projections.10 This group, which was established in 1999 and is made up of academics, researchers and public health practitioners, has led the development of several modelling and estimation packages (for example, the Workbook, EPP and the AIM module in Spectrum) that are used by countries and UNAIDS and the World Health Organization in preparing national estimates. The group meets yearly to review the major themes providing inputs to the estimation …