OBJECTIVE: To derive methods to estimate and project the fraction of children orphaned by AIDS and other causes. METHODS: HIV/AIDS affects orphan numbers through increased adult and child mortality and reduced fertility of HIV-positive women. We extend an epidemiologic and demographic model used previously to estimate maternal orphans to paternal orphans. We account for the impact of HIV/AIDS on child survival by modeling the HIV status of the partners of men who die of AIDS or other causes based on data on the concordance of heterosexual partners. Subsequently, the proportion of orphans whose parents have both died is predicted by a regression model fitted to orphanhood data from 34 national demographic and health surveys (DHSs). The approach is illustrated with an application to Tanzania and compared with DHS estimates for the years 1992 and 1999. RESULTS: Projections of the number and age distribution of orphans using these methods agree with survey data for Tanzania. They show the rise in orphanhood over the last decade that has resulted from the HIV epidemic. CONCLUSIONS: The methods allow estimation of the numbers of children whose mother, father, or both parents have died for countries with generalized heterosexual HIV epidemics. These methods have been used to produce orphan estimates for high-prevalence countries published by Joint United Nations Program on HIV/AIDS, World Health Organization, United Nations Children's Fund, and US Agency for International Development in 2002 and 2004.