PEG‐ADA is a long‐circulating form of adenosine deaminase (ADA) that has been in use for <8 years as replacement therapy for severe combined immunodeficiency disease due to ADA deficiency. Treatment with PEG‐ADA almost completely corrects metabolic abnormalities, allowing the recovery of a variable degree of immune function. Although not normal, the level of function achieved has in most cases been sufficient to protect against opportunistic and life‐threatening infections. PEG‐ADA has been used as an alternative for patients who lack an HLA‐identical bone marrow donor, but are judged to be at too high a risk for undergoing HLA‐haploidentical marrow transplantation. To date, mortality and morbidity with PEG‐ADA have been less than for the latter procedure. PEG‐ADA has also been an important adjunct to attempts to develop somatic cell gene therapy for ADA deficiency, although its continued use poses a problem for evaluation of the benefit of gene therapy. As a true “orphan drug” developed to treat a very small patient population, the cost per patient of PEG‐ADA is very high. © Wiley‐Liss, Inc.