During the past 6 y, 29 adenosine deaminase (ADA)-deficient patients with combined immunodeficiency have been treated with polyethylene glycol (PEG)-modified bovine ADA (PEG-ADA). We have monitored plasma ADA activity, metabolic effects of treatment, and the evolution of antibody to PEG-ADA in these patients, in collaboration with immunologists and clinicians in North America, Europe, and Australia, who have monitored immune function and clinical response to treatment. This article summarizes the current status of PEG-ADA therapy and provides recommendations for its use. Recovery of specific immune function during treatment with PEG-ADA is illustrated for three patients, who represent early, delayed, these patients have entered a trial of gene therapy, but continue to receive enzyme replacement.