The prevalence of human immunodeficiency virus (HIV) among adults and mortality rates among under-5-year-olds have increased or stagnated in many countries. The objective of this study was to investigate whether there is a link between under-5 mortality trends and the prevalence of HIV among adults and, if so, to assess the magnitude of the effect of adult HIV prevalence on under-5 mortality rates.Data from Demographic and Health Surveys were used to establish the trends in under-5 mortality rates for 25 countries for which there are data for at least two points in time. Countries were ranked according to the most recent adult HIV prevalence data and grouped in three categories: those with very high HIV prevalence (> or = 5%); those with moderately high prevalence (1-4.9%); and those with low prevalence (< 1%). A mathematical model was fitted to obtain an estimate of the contribution of HIV/AIDS to the level of under-5 mortality in each country.Under-5 mortality rates showed an increase in most countries with high adult HIV prevalence, but a decrease in almost every country with moderately high or low prevalence. The estimated contribution of adult HIV prevalence to the observed level of under-5 mortality was highest (up to 61%) in Zimbabwe (where HIV prevalence was highest) and tended to decrease with the level of HIV prevalence.The contribution of HIV/AIDS to childhood mortality therefore appears to be most noticeable in settings where the epidemic is most severe.