Although β thalassemia is considered to be a classic monogenic disease, it is clear that there is considerable clinical variability between patients who inherit identical β globin gene mutations, suggesting that there may be a variety of genetic determinants influencing different clinical phenotypes. It has been suggested that variations in the structure or amounts of a highly expressed red cell protein (alpha hemoglobin stabilizing protein [AHSP]), which can stabilize free α globin chains in vitro, could influence disease severity in patients with β thalassemia. To address this hypothesis, we studied 120 patients with Hb E-β thalassemia with mild, moderate, or severe clinical phenotypes. Using gene mapping, direct genomic sequencing, and extended haplotype analysis, we found no mutation or specific association between haplotypes of AHSP and disease severity in these patients, suggesting that AHSP is not a disease modifier in Hb E-β thalassemia. It remains to be seen if any association between AHSP and clinical severity is present in other population groups with a high frequency of β thalassemia. (Blood. 2004;103:3296-3299)