The validity of UNAIDS/WHO estimates of the burden of HIV/AIDS is rightly questioned by politicians, scientists, and activists-especially since the 2003 estimates to be released in July, 2004, will show substantial drops in the burden of HIV/AIDS in several countries, and increases in others. However, the estimates are based on an explicit attempt to meet criteria we believe should guide the generation of international morbidity and mortality figures. These criteria extend beyond the quality of the input data to include features of the estimation process such as transparency and participation. The 2003 estimates now include plausible ranges for estimates rather than a single best estimate. This reduces the chance that insignificant differences in estimates from different sources are given importance. Here, we describe the levels of uncertainty associated with the UNAIDS/WHO estimates of HIV/AIDS. We explain the reason for moving to the use of plausibility bounds, the factors that determine the width of the bounds, and the implications for policy makers and programme managers.