BACKGROUND: Despite the 15 million children orphaned by AIDS, and fears of sexual vulnerability, little is known about the link between orphanhood and HIV risk. METHODS: A random sample of 1283 15 to 19-year-old girls in a high-density suburb of Harare was identified in a cross-sectional survey in 2004. A total of 863 agreed to be interviewed and 839 provided a specimen for HIV and herpes simplex virus type-2 (HSV-2) testing. Sexual health outcomes, sexual behaviours and marriage were assessed by type and timing of orphanhood. RESULTS: Half of the participants were single or double orphans. Prevalence of HIV and/or HSV-2 was higher among orphans than non-orphans [17 versus 12%; age-adjusted odds ratio (aOR) = 1.5; 95% confidence interval (CI) 1.0-2.3]. Associations with orphan status were only significant among the 743 never-married participants. In comparison with non-orphaned peers, increased sexual risk (defined as HSV2-positive, HIV-positive or ever pregnant) was seen among maternal orphans (aOR = 3.6; 95% CI, 1.7-7.8), double orphans (aOR = 2.4; 95% CI, 1.2-4.9), and girls who lost their father before age 12 (aOR = 2.1; 95% CI, 0.9-4.8) but not later (aOR = 0.8; 95% CI, 0.3-2.2). Maternal and double orphans were most likely to initiate sex early and to have had multiple partners. Maternal orphans were least likely to have used a condom at first sex, and to have a regular sexual partner. Experience of forced sex was high in all groups. CONCLUSIONS: In urban Zimbabwe, female adolescent orphans are at increased risk of HIV and HSV-2 infection. Infection rates vary by type and age of orphanhood, and marital status, and are associated with high-risk sexual behaviours.