Soil transmitted helminths infections can have a negative impact on the health of children affecting nutritional status and development. Therefore, most endemic countries have started implementing mass chemotherapy programmes through school infrastructures. The success of a national deworming programme may be influenced by environmental conditions or the access to water, sanitation and hygiene. The question is which of these factors lead to success or failure of a programme. We therefore tried to get a better understanding of the determinants of geographical variations in programme impact in Kenya.