['To determine prevalence and contributing factors of anaemia in adolescents.', 'Cross-sectional study of anaemia prevalence, socio-economic status and puberty.', 'Schools in East Java, Indonesia.', 'Male and female adolescent pupils (age 12-15 y; n=6486).', 'Anaemia prevalence was 25.8% among girls (n=3486), 24.5% among pre-pubertal boys (n=821), and 12.1% among pubertal boys (n=2179). Socio-economic status, indicated by type of school attended, was an important factor determining the risk of anaemia. Girls had a higher risk when they attended a poor school (OR poorest school, 1.00; other schools, 0.67-0.87), had reached puberty (OR, 1.25), had lower retinol intake (OR 1st-4th quartiles-1.00, 0.97, 0.89, 0.77) and higher vitamin A intake from plant sources (OR 1st-4th quartiles-1.00, 1.10, 1.31, 1.04). Boys had a higher risk of anaemia when they attended a poor school (OR poorest school 1.00, other schools 0.54-0.63), were younger (OR per year=0.79), had not yet reached puberty (OR not yet, 1.00; already, 0.78), were shorter (OR per cm 0.95), had smaller mid-upper-arm circumference (MUAC) (OR per mm 0.99) and lower retinol intake (OR 1st-4th quartile 1.00, 0.67, 0.74, 0.68).'] Are socio-economic status and puberty the main factors determining anaemia in adolescent girls and boys in East Java , Indonesia?