Phenotypic characterization of Diamond Blackfan Anemia (DBA) patients and their relatives was performed in 54 families. Complete blood count, fetal hemoglobin level, erythrocyte i antigen expression, and erythrocyte adenosine deaminase (eADA) activities were quantitated in patients and relatives. eADA was elevated in 28 of 34 transfusion-independent DBA patients, whereas persistence of erythrocyte i antigen was noticed in only 10 of 20 DBA patients. High eADA activities were also found in 14 of 149 healthy family members, allowing us to identify an isolated high eADA phenotype in these families. In contrast, increase in erythrocyte i antigen expression, elevated fetal hemoglobin levels, and macrocytosis were much less frequently noted in nonaffected members of the DBA families studied. Importantly, isolated high eADA phenotype was found to be significantly associated with genetic markers on chromosome 19 that segregate with the DBA phenotype. Isolated high eADA phenotype thus seems to reflect a silent phenotype of DBA in affected families. These findings suggest that elevated eADA activity in unaffected individuals needs to be taken into account during genetic assessment of DBA families and could be used for accurate assessment of mode of inheritance.