beta-Thalassemia (thal) is relatively common in Bulgaria. Over the past 40 years population studies have been carried out in most parts of the country. Different approaches for the detection of beta-thal carriers were used and a frequency from 0.5 to 19.9% was found. We have been studying beta-thal in Bulgaria since 1965 and, based on our results, the average frequency is 2.5%. Here we update our results on the molecular basis of beta-thal and include some unpublished data. One thousand seven hundred and fifty-two patients with signs of hemolysis were studied. Among these, 723 patients (41.3%) had beta-thal or a related condition. In addition, blood samples from 875 newborn babies were studied. Eighteen different beta-thal alleles were identified. The codon 39 (C-->T) and IVS-I-110 (G-->A) mutations occurred most frequently, and seven additional mutations were observed that were present at frequencies of 2.4 to 14.2%. This broad spectrum of beta-thal alleles complicates the analysis for institutions involved in prenatal diagnosis. The frequency of alpha-thal is low (0.5% alpha-thal-1 and 1.6% for alpha-thal-2).