The United Nations children's fund, Unicef, has warned in its annual report that international development efforts are drastically short-changing girls.
By Imogen Foulkes
BBC correspondent in Geneva
It says millions are left out of school every year and this, the organisation says, has serious consequences, not just for the children themselves but for their families, their communities and their countries.
Unicef says educating girls is crucial for the future of developing countries
Every year, 120 million children are left out of primary school. Most of them are girls.
Campaigns for universal education have not benefited girls and boys equally, Unicef says - the gender gap is still too wide.
In many countries, girls are last on the list for a place in school and the first to leave when times get hard.
Worst-affected are sub-Saharan African and South and East Asia, where the number of girls not in school is actually rising, with devastating effects.
Countries with the fewest number of girls in secondary school have the highest rates of child mortality.
Illiteracy among women far higher than men - 9 million more girls than boys are left out of school every year
Low enrolment for girls is linked to higher rates of child mortality
83% of all girls out of school live in sub-Saharan Africa, South Asia, and East Asia and the Pacific
Denying girls an education makes them more vulnerable to poverty and hunger; they are more likely to die in childbirth and they are at greater risk of diseases like HIV-Aids.
Conversely, girls who do get an education have healthier children and are more likely to send their own daughters and sons to school.
Unicef says educating girls is crucial for the future of developing countries.
It has set a goal of eliminating barriers to education in 25 target countries by the year 2005.
But, Unicef warns, wealthy countries have failed to meet their commitments to education in the developing world.
The report calls for a series of measures to improve things, including increased international funding, the elimination of all school fees and the inclusion of education as an essential element in development projects.