In this study, 32 patients with Hb H (beta 4) disease have been identified. Three different alpha-thalassemia-1 (thal) determinants; nine with the -17.4 kb (MED I) type, 12 with the -20.5 kb type and 10 with the -26.5 kb (MED II) type were characterized. Of the 32 patients, 19 had the 3.7 kb deletion and one had the 4.2 kb deletion in trans to alpha-thal-1 determinants. Only one patient, homozygous for the polyadenylation signal (poly A) site (PA 1) mutation, was identified to be associated with Hb H disease. The other patient had the poly A (PA 1) mutation in trans to the MED I (-17.4 kb) determinant. The 5 nt (nucleotide) deletion was present in three patients, two of them in the same family, this mutation was found in association with the MED II (26.5 kb deletion). The other patient had the -5 nt mutation in trans to the MED I (-17.4 kb) determinant. An unstable hemoglobin (Hb) variant [Hb Adana, codon 59 (C -> A)] was present in association with the alpha-thal-1 deletion (20.5 kb) in two adults and caused a severe type of Hb H disease. Five patients with TA H disease had the genotype - -(MED II)/alpha(PA 2) cr one had a Hb S heterozygosity (- -(MED II)/ alpha(PA 2)alpha + Hb AS). A patient with Hb H disease (- -(MED I)/-alpha(3.7)) also had Hb S trait.