In this study we have defined the molecular basis and correlated the clinical phenotype with the α-globin genotype in a large series of patients of Sardinian descent with HbH disease. The most prevalent molecular defect was the deletion of 3 α-globin structural genes most commonly the (– –/–α^3.7) genotype (83.6%) and rarely the (– –/– α^4.2) genotype (1.4%), followed in decreasing order of incidence by the combination of deletion α°-thalassemia and initiation codon mutation of the α₂-gene (– –/α^NcoIα = 9.8%), deletion α°-thalassemia and pentanucleotide deletion of IVS-I of the α₂-globin gene, (– –/α^HphIα = 3.3%) deletion α°-thalassemia and initiation codon mutation of the α₁-gene (– –/αα^NcoI = 1.3%), a homozygous state for initiation codon mutation of the α₂-gene (α^Ncoα/α^NcoIα = 0.7%). Patients with the (– –/α^thalα) genotypes showed severer clinical and hematological features as compared to those with the (– –/–α) or those with the (– –/αα^thal) genotypes. The single patient with the (α^Ncoα/α^Ncoα) genotype had a clinical phenotype intermediate between HbH disease and the α-thalassemia carrier status. This heterogeneity depends on the fact that the α₂-globin gene produces 2-3 times α-globin chains than the α₁-gene and the single remaining α₁-like globin gene in the -α^{3 7} chromosome has a compensatory increase in the α-globin chain output. α-Globin gene mapping of HbH disease patients may be useful for predicting the clinical outcome and to improve genetic counselling.