Little is known about the selenium status of children living in the Andean regions of South America, which commonly have volcanic soil with low selenium content. Human selenium deficiency has been hypothesized to have a negative impact on immune function and to increase the risk of infection. The objective of this study was to evaluate the serum selenium concentrations of severely malnourished children living in urban and rural Andean Ecuador, and to compare them to a control group of normally nourished children from the same communities. Forty-three children, aged six to 36 months, with marasmus or kwashiorkor and 30 control children were enrolled from July to November 1993 in Quito, Ecuador. Serum selenium concentrations were lower in the children with marasmus (0.91 ± 0.28 muM/L, n = 21) and kwashiorkor (0.37 ± 0.15 muM/L, n = 22) than in those who were normally nourished (1.77 ± 0.75 muM/L, n = 30; p < 0.001 for each difference). The serum selenium concentrations in children with kwashiorkor were significantly lower than those in children with marasmus (p < 0.001). All 22 of the children with kwashiorkor, 15 of the 21 children with marasmus, and five of the 30 normal children had serum levels < 1.08 muM/L (8.5 mug/dL) (chi2 = 38.4, p < 0.00000001). In the Andean regions of Ecuador, selenium deficiency is prevalent in children with protein and caloric deficiency. Furthermore, 17% of Ecuadorian children with normal weight-for age-Z score are selenium-deficient.