Diarrhoea is one of the major causes of childhood death in developing countries. In this research project, we studied factors that presumably affect the incidence of diarrhoea among young children in urban slums: consumption of safe drinks, hygiene behaviour, cleanliness of household surroundings and the quality of raw water. Beliefs concerning the causes of diarrhoea were also related to health-improving behaviour, namely the application of the water-treatment method SODIS (solar water disinfection) and hygiene behaviour. We conducted a survey in Kibera, a huge shantytown in Nairobi, Kenya. Field workers interviewed a random sample of 500 households. Analysis with regression models revealed that two out of the four postulated factors (mentioned above) were significant: children have a lower risk of contracting diarrhoea when they consume high percentages of safe drinks and live in households with good hygiene behaviour. As regards beliefs, we found that biomedical knowledge of children’s diarrhoea as well as the perceived social norm for treating water are associated with the use of SODIS and good hygiene behaviour. These findings underline the importance of water treatment at household level and safe hygiene practices for protecting the health of young children growing up in urban slum settlements. Our data suggest that these measures are more important for diarrhoea prevention than efforts focussing on cleaning up neighbourhoods.