Probable homozygous beta thalassemia was detected in a Negro child. The criteria for this diagnosis included (1) a compatible clinical picture, (2) 60% hemoglobin F, 8.8% hemoglobin A2, and 31.2% hemoglobin A, and (3) peripheral blood smear compatible with homozygous beta thalassemia. The mild clinical course in this patient and in other Negroes with this disease reported, is suggestive of a fundamental difference between the defect in Negroes and other ethnic groups. A review of the literature shows the paucity of this disease in Negroes, with only eight other cases reported in America. The reasons for this are unknown. Patients with homozygous beta thalassemia who show a mild clinical course serve as an excellent example of how genetic modifiers are involved in the various forms of this entity.