Polyethylene glycol-modified adenosine deaminase (PEG-ADA) has now been used for 8.5 years as enzyme replacement therapy for immunodeficiency due to ADA deficiency. PEG-ADA restores a metabolic environment necessary for recovery of immune function. In most cases, the level of function achieved has been sufficient to protect against opportunistic and life-threatening infections. To date, mortality and morbidity with PEG-ADA have been less than for haploidentical bone marrow transplantation. As a true "orphan drug" used to treat a very small patient population, the cost per patient of PEG-ADA is very high, but it has been well tolerated, free of adverse reactions, and effective as an alternative for patients who lack an HLA-identical marrow donor, but are considered too ill to undergo haploidentical marrow transplantation. Concomitant treatment with PEG-ADA has also permitted investigation of gene therapy to be carried out safely.