A review is presented of the various beta-thalassemia alleles observed in nearly 191 patients with beta-thalassemia major and their 182 heterozygous relatives. Determination was by gene amplification and dot-blot hybridization with synthetic probes, specific for 27 different mutations. Eighteen mutations have been observed; six of these account for nearly 83% of all thalassemia abnormalities. A new mutation, i.e. a G----C mutation at the acceptor splice site of IVS-I, was found in one teenager who was homozygous for this disease. The high consanguinity among the families was considered the main reason for the high number of patients with a homozygosity for the IVS-I-110 (G----A) mutation. Combinations of different mutations were present in many patients; some were mildly affected because of the specific mutation present on one chromosome. Combinations of classical beta-thalassemia and an abnormal hemoglobin mainly concerned Hb S. Hbs Knossos and Lepore were rare occurrences. A comparison of hematological data for adults with heterozygosities for some of the common alleles confirmed the low Hb A2 values in IVS-I-6 (T----C) heterozygotes and the high Hb F values for codon 8 (-AA), IVS-II-1 (G----A), and IVS-I-1 (G----A) heterozygotes.