The study explores the relationship between the sickle-cell trait and malaria infection. It is found that the sickle-cell trait provides significant protection against malaria in indigenous East Africans. The incidence of parasitemia in 43 Ganda children with the sickle-cell trait was significantly lower than in a comparable group of 247 children without the trait. An infection with P. falciparum was established in 14 out of 15 Africans without the sickle-cell trait, whereas in a comparable group of 15 Africans with the trait only 2 developed parasites. It is concluded that the abnormal erythrocytes of individuals with the sickle-cell trait are less easily parasitized by P. falciparum than are normal erythrocytes. Hence those who are heterozygous for the sickle-cell gene will have a selective advantage in regions where malaria is hyperendemic. This fact may explain why the sickle-cell gene remains common in these areas in spite of the elimination of genes in patients dying of sickle-cell anaemia. The implications of these observations in other branches of haematology are discussed.The study explores the relationship between the sickle-cell trait and malaria infection. It is found that the sickle-cell trait provides significant protection against malaria in indigenous East Africans. The incidence of parasitemia in 43 Ganda children with the sickle-cell trait was significantly lower than in a comparable group of 247 children without the trait. An infection with P. falciparum was established in 14 out of 15 Africans without the sickle-cell trait, whereas in a comparable group of 15 Africans with the trait only 2 developed parasites. It is concluded that the abnormal erythrocytes of individuals with the sickle-cell trait are less easily parasitized by P. falciparum than are normal erythrocytes. Hence those who are heterozygous for the sickle-cell gene will have a selective advantage in regions where malaria is hyperendemic. This fact may explain why the sickle-cell gene remains common in these areas in spite of the elimination of genes in patients dying of sickle-cell anaemia. The implications of these observations in other branches of haematology are discussed.