There is increasing research and programmatic attention to vulnerable and marginalized adolescents in developing countries. However, very little information exists on adolescent domestic workers, which may constitute a substantial proportion of urban girls in some settings. This is a population-based study of adolescents aged 10-19 years in low-income and slum areas of Addis Ababa, Ethiopia. Descriptive analysis was conducted to compare female domestic workers with other adolescent girls and boys in terms of background, working patterns, self-esteem and social connections, and exposure to HIV and adolescent programmes. Fifteen per cent of the female adolescent population were domestic workers, most of whom had migrated from rural areas. Domestic workers were less likely to be educated or to live with parents compared to other categories of adolescents. They worked extremely long hours for low pay, with a mean income of US$6 per month. Domestic workers appeared to have lower self-esteem and fewer friends than other adolescents, as well as lower levels of HIV knowledge and minimal participation in existing adolescent programmes. Adolescent domestic workers are highly vulnerable yet largely invisible, even despite their large numbers in some urban settings. Greater programmatic attention and awareness-raising needs to be devoted to this large, at-risk group of adolescent girls.