OBJECTIVE: To quantify and describe orphan incidence in Manicaland, eastern Zimbabwe. DESIGN: Open cohort study. METHODS: Statistical analysis of data on 13,740 and 10,308 children, aged 0-14 years, enumerated in household censuses in four socio-economic strata, 1998-2000 and 2001-2003, and 10,184 children seen in both censuses (74% follow-up). RESULTS: Prevalence of all forms of orphanhood increased. The overall rate of losing a parent amongst non-orphans was 27.5 per 1000 person-years (py). Paternal orphan incidence (20.2 per 1000 py) was higher than maternal orphan incidence (9.1 per 1000 py) and maternal orphans lost their fathers at a faster rate than paternal orphans lost their mothers. Paternal and maternal orphan incidence increased with age. Incidence of maternal orphanhood and double orphanhood amongst paternal orphans rose at 20% per annum [incidence rate ratio (IRR) = 1.20; 95% CI, 1.06-1.35] and 71% per annum (IRR = 1.71; 95% CI, 1.25-2.33), respectively, 1998-2003, but incidence of paternal orphanhood and double orphanhood amongst maternal orphans were unchanged. For 82% of children with a parent who died, the parent was HIV-positive at baseline. More new paternal and double orphans--but not new maternal orphans--than non-orphans had left their baseline household. Mortality was higher in orphans than non-orphans with the highest death rates observed amongst maternal orphans. CONCLUSIONS: Orphan incidence and prevalence are high and increasing due to HIV in eastern Zimbabwe. Orphan incidence patterns differ from orphan prevalence patterns and need to be understood if support programmes are to assist children during periods of high vulnerability.