The prevalence of visceral leishmaniasis and malaria in the human population of West Pokot district of Kenya was studied in 1986. A total of 2139 people was proportionately screened for the two diseases according to four age categories (0-4, 5-14, 15-44 and greater than 45 years). Diagnostic methods included the enzyme linked immunosorbent assay (ELISA) and Leishmanin skin test for visceral leishmaniasis, and parasitological examination for malaria. The epidemiological value of the spleen rate was evaluated in relation to visceral leishmaniasis and malaria endemicity. A general decline of infection rates with altitude was observed for both diseases. Visceral leishmaniasis was less prevalent than malaria, with less than 2% active cases in any age group and had the same distribution in both sexes. Malaria infection rate was highest in the younger age groups, declining from 21.5% in the 0-4 year old age group to 5.5% in people more than 45 years old. Malaria affected significantly more males than females. The spleen rate was inappropriate for epidemiological survey of either malaria or visceral leishmaniasis due to an overlap in the distribution of the two diseases.