The structure, properties, genetics, and clinical and biochemical expression of hemoglobins Lepore (δβ) and anti-Lepore (βδ) are described. In addition to the three Lepore variants (Lepore Hollandia, Lepore Baltimore and Lepore Washington) at least four anti-Lepore variants (Miyada, P Nilotic (P Congo), Coventry and Lincoln Park) are known at the present time. All known hemoglobins Lepore and anti-Lepore are products of non-homologous crossing-over between theδand theβgenes. Although the Hb Lepore condition is expressed phenotypically and clinically asβthalassemia, the presence of about 10% of Hb Lepore distinguishes the condition hematologically fromβthalassemia. Data on the hematological and biochemical expression of this hemoglobinopathy are presented. In contrast to the anemia in the Lepore condition, there is no phenotypic evidence of thalassemia in persons with hemoglobin anti-Lepore, because noβchain deficiency accompanies the latter condition. Although no adequate explanation has been advanced concerning the factors which maintain a low synthesis of the Lepore and anti-Lepore chains, it has been suggested that multiple rare codons may introduce rate-limiting steps or that theδβandβδmRNAs may be unstable. Data on the geographical distribution and structural identification of Hb Lepore are presented.