No data has been published, so far about the prevalence of cryptosporidiosis among children in Niger; a landlocked country of West Africa where malnutrition and diarrhoea are two major public health issues. The aim of the present study was to get a first evaluation of the prevalence of Cryptosporidium sp in the stools of hospitalized children in the National Hospital of Niamey (NHN) where we carried out a prospective descriptive study involving all children younger than 5 years of age in the paediatric department between February 21st and May 22nd, 2004. Direct stool examination and Ritchie technique were systematically performed, and evidence of Cryptosporidium oocysts was looked for by means of a smear from the sediment which was stained by the modified Ziehl-Neelsen technique. The weight/age ratio was calculated and analyzed with the Epi-Info software, based on the reference population defined by the US National Center for Health Statistics (NCHS). Malnutrition was defined as a weight/age ratio more than 2 SD below the NCHS's reference population. Malnutrition was considered moderate between -2 and -3 SD and severe below 3 SD. In the 3 months study 220 children were included (sex ratio = 1.18 and mean age = 20 months) showing that 65% of the children were suffering from malnutrition (moderate = 17.3%, severe = 47%). Diarrhoea was reported in 51.8% of the children. Cryptosporidium oocysts were detected in 12/220 children (5.5% of the studied population) and 7/114 (6. 1%) of those children were suffering from diarrhoea. 10 (83%) out of the 12 infected children were malnourished. This was the first study ever conducted in the paediatric department of the Niamey hospital, and it showed evidence of a 5.5% prevalence of cryptosporidiosis in the overall studied population, versus 6.1% among children with diarrhoea. 5 children without diarrhoea and 2 children under 6 months were also infected. Another study based on a larger number of patients would be necessary to address the impact of rainfall distribution on the incidence of the disease.