We undertook a representative survey of measles antibodies in Addis Ababa, Ethiopia 1994, to characterize immunity and transmission. Specific-antibody levels (IU/l) were determined by ELISA for 4654 sera from individuals aged 0–49 years (1805 <15 years) collected by stratified household-cluster sampling. The proportion seronegative (<100 IU/l) was 20% (95% CI: 16–25) in children 9–59 months old, declining to 9% (7–12) in 5–9 year olds, 5% (4–7) in 10–14 year olds, and <1% in adults. The proportion of children (<15 years old) with low-level antibody (100–255 IU/l) was 8% (7–10). Vaccination and an absence of a history of measles illness were strongly associated with low-level antibody. History of measles vaccination in 9 months to 14-year-old children was ~80%. We estimate a primary vaccine failure rate of 21% (12–34) and continued high measles incidence of 22 per 100 susceptibles (19–24) per annum. Our data support the introduction of campaign vaccination in the city in 1998, although higher routine vaccine coverage is required to sustain the impact. The implications of a high prevalence of low-level antibody are discussed.