Pneumonia and diarrhoea, both caused by bacteria, are the two biggest causes of childhood deaths in the world, killing more than 3.5 million children every year.
Doctors led by Stephen Luby, of the US Centers for Disease Control and Prevention (CDC) carried out a trial in a squatter settlement in the Pakistani city of Karachi, according to the report in Saturday's issue of the medical journal Lancet.
Six hundred households were assigned to a scheme to promote handwashing.
They were given either anti-bacterial soap or ordinary soap, and were visited by fieldworkers at least once a week for a year, who distributed the soap, encouraged handwashing and recorded the state of health of household members.
Three hundred other households were "controls" - they acted as a benchmark, receiving neither soap nor fieldworker visit.
In the "soap" group, incidence of pneumonia fell by 50% percent, prevalence of diarrhoea declined by 53% and cases of the bacterial skin disease impetigo were reduced by 34% percent in children aged under 15, as compared to the no-soap group.
There was no difference in results between anti-bacterial or plain soap.
"Our data show that regular handwashing with soap is very effect in preventing...two of the leading causes of global childhood death," Luby says.
"Our data show that regular handwashing with soap is very effect in preventing...two of the leading causes of global childhood death"
US Center for Disease Control and Prevention
He notes, however, that buying the soap and providing community support to encourage handwashing can still be a heavy burden in very poor countries.
In Karachi, for instance, the soap itself cost around $1 a week.
This seems modest but it has to be seen in the context of the residents' income: the average weekly household income was less than $15, which means that many families gave spending priority to food.
"The challenge for the public health community is to identify cost-effective techniques for handwashing promotion that can reach hundreds of millions of households at risk," Luby notes.