beta-thalassemia and sickle cell disease (SCD) are common disorders in Turkey. Compound heterozygosity for these two disorders (beta(S)/beta-thalassemia) is encountered frequently. In this report we present hematological and molecular data of two Turkish siblings with beta(S)/beta(del)-thalassemia caused by a 290 base pair (bp) deletion and associated with increased levels of hemoglobin A(2) (HbA(2)) and hemoglobin F (HbF). Clinical analysis of the two patients showed a mild course of the disease. Haplotypic factors involved in increasing the levels of HbF were analyzed. The two patients showed no changes from the normal sequences at the Xmnl site of G gamma-globin promoter and the (AT)(x)T-y microsatellite 5' to the beta-globin mRNA cap site, The removal of the region between positions -125 to +78 relative to the beta-globin gene mRNA cap site by the 290 bp deletion is thought to allow the beta-locus control region to interact with the promoters of the delta- and gamma-globin genes, leading to increased HbA(2) and HbF levels. (C) 1998 Wiley-Liss, Inc.