Inadequate sanitation is one of the biggest contributing factors to child mortality under the age of five and yet it remains the most neglected of the Millennium Development Goal (MDG) sectors. Every year, 10 million children die before reaching their fifth birthday. This paper asserts that improved sanitation could bring a significant reduction in these deaths, and secure progress on MDG4. Recent research in Brazil has shown that access to improved sanitation alone reduced the rate of childhood diarrhoea by up to 43% and handwashing with soap at critical times alone has been shown to reduce the incidence of diarrhoea by 45%. The failure to increase access to sanitation acts as a brake on development and makes the realisation of broader development outcomes both unlikely and unsustainable. © 2009.