ABSTRACTThe effect of red cell transfusion and polyethylene glycol‐modified adenosine deaminase therapy on biochemical abnormalities, clinical status, and immunologic function in an adenosine deaminase‐deficient child was investigated. After red cell transfusions, erythrocyte deoxyadenosine triphosphate (dATP) concentrations decreased about 95% and were closely related to adenosine deaminase activities; deoxyadenosine diphosphate concentrations decreased only approximately 30%. The evolution of dATP levels was also closely related to the improvement in clinical status of the patient. However, immune function was not restored. After polyethyelene glycol‐modified adenosine deaminase therapy, the concentration of erythrocyte dATP decreased to undetectable levels correlated with an increase of T lymphocyte counts and an increase of lymphocyte responses to mitogens. Immune functions were restored only when dATP levels were below 15 &mgr;mol/L. It appears that red cell transfusion therapy is not sufficiently effective to reduce and maintain erythrocyte dATP levels at values compatible with normal immune function. On the contrary, polyethylene glycol‐modified adenosine deaminase therapy is a suitable treatment to reduce dATP levels to near undetectable values, allowing the immune function to be restored. dATP measurement is a very useful tool for monitoring and evaluating the degree of efficiency of therapy in adenosine deaminase deficiency. (Pediatr Res28: 127‐130, 1990)AbbreviationsADA, adenosine deaminasedADP, deoxyadenosine diphosphatedATP, deoxyadenosine triphosphatePEG‐ADA, polyethylene glycol‐modified adenosine deaminaseSAH, S‐adenosyl homocysteineSCID, severe combined immunodeficiency disease