The degree of anemia in beta(0)-thalassemia/hemoglobin E disease is highly variable. As part of an attempt to identify determinants of this variability of severity we studied concordance and discordance of hemoglobin levels among sib pairs. The distribution of differences of hemoglobin levels in 216 sib pairs from 98 families showed a remarkable skewness toward the lower values with a mode at 0-0.5 gm/dl. The prevailing concordance of hemoglobin levels in patients from the same families and the persistence of the patterns indicate that polygenic factors are mainly responsible for the variability of anemia in this disease.