Public-health researchers led by Mauricio Barreto of the Federal University of Bahia examined children in the city of Salvador before and after a sanitation system was installed in the late 1990s, boosting sewerage coverage from 26 percent of households to 80 percent.
Incidence of diarrhoea fell on average by 22 percent with a reduction of 44 percent in areas where this disease was most widespread, according to the sample of more than 1,800 children.
Barreto's team says the findings show that sanitation must be given equal ranking alongside adequate water supply in policies to promote public health in poor countries.
An estimated 1.6 billion people will need access to improved sanitation by 2015 to meet the United Nations' Millennium Development Goal (MDG) on this issue. The UN has declared 2008 to be the International Year of Sanitation to draw attention to the problem.
The study appears in next Saturday's issue of the British medical weekly.