Hb Southampton (also known as Hb Casper) is characterized by the substitution of a leucine residue for a proline at codon beta106 (CTG-->CCG). This mutation breaks the G helix and severely distorts the tertiary structure of the molecule, producing an unstable hemoglobin (Hb) and severe hemolysis. We identified this hemoglobinopathy in a young patient with severe hemolytic anemia and hepatosplenomegaly.