alpha-Thalassemia is among the world's most common single gene disorders, which is most prevalent in the malaria belt. This geographic distribution has been attributed to a selective advantage of heterozygotes against this disease. Unexpectedly, we have found a high frequency of heterozygosity for deletional alpha-thalassemia (-alpha3.7) in Ashkenazi Jews (carrier frequency of 7.9%, allele frequency of 0.04). This population has resided in temperate climates for many centuries and was therefore not subjected to malarial selection pressure. In comparison, heterozygosity for beta-thalassemia, which is highly subject to malarial selection pressure, is very low (estimated <0.1%) in this group. It is possible that founder effect and genetic drift have contributed to the high frequency of deletional alpha-thalassemia in Ashkenazim, as may occur in closed populations. Alternatively, we hypothesize that positive selection pressure for an as yet unknown linked allele on chromosome 16 may be a significant factor leading to this high frequency.