Four main conditions resulting from deletion or inactivation (nondeletion mutants) of one, two, three, or all four alpha-globin genes are recognized. Carriers of alpha0-thalassemia (two deleted alpha-globin genes, ie, alpha-thalassemia trait) show microcytosis, hypochromia, and normal percentages of HbA2 and HbF, carriers of alpha+-thalassemia (one deleted or nonfunctional alpha-globin gene, ie, alpha-thalassemia silent carrier) have either a silent hematologic phenotype or present with a moderate thalassemia-like hematologic picture. Two are the alpha-thalassemia clinically significant forms: Hb Bart hydrops fetalis syndrome and HbH disease (four and three deleted or nonfunctional alpha-globin genes, respectively). Hb Bart hydrops fetalis syndrome is the most severe form, characterized by fetal onset of generalized edema, pleural and pericardial effusions, and severe hypochromic anemia, in the absence of ABO or Rh blood group incompatibility. Death usually occurs in the neonatal period. HbH disease is characterized by microcytic, hypochromic hemolytic anemia, hepatosplenomegaly, mild jaundice, and sometimes thalassemia-like bone changes. Detection of red blood cell inclusion bodies (precipitated HbH) with supravital stain and HbH by hemoglobin analysis with high-performance liquid chromatography (HPLC) or electrophoresis is diagnostic for HbH disease. Genetic testing is used to confirm hematologic and clinical diagnosis and is useful in carriers for genetic counseling. If both parents carry an alpha0 mutation, they have a risk of 25% of having an offspring with Hb Bart syndrome at each conception. In patients with HbH, genetic testing has a prognostic value, as interactions between nondeletional molecular defects or between deletional and nondeletional defects result in more severe phenotypes than interactions with deletional molecular defects.