The frequency of the βs mutation in the district of Coruche/Portugal is estimated to be about 4% from analysis of a group of 181 school children and their teachers in an area in which malaria has been endemic until recently. Several white Portuguese patients with sickle cell disease (six homozygous SS and one Sβ° thalassaemia) were found in a group of 309 further patients who were known and followed up by local medical practitioners. These patients had clinical and haematological features similar to patients of African origin, although their growth and sexual development appeared to be normal. The analysis of an array of polymorphic restriction sites within the βs globin gene cluster (βs haplotype) showed patterns that are known to occur in Africa. The frequencies of the three main African βs haplotypes termed Senegal, Bantu, and Benin reflect the extent of Portuguese naval explorations. It is concluded that the sickle cell gene in Portugal has probably been imported from Africa and has been amplified in comparison with other genes characteristic for African races because of the selective advantage of AS heterozygotes in an area endemic for malaria.