Adenosine deaminase (ADA) deficiency is one the causes of severe combined immunodeficiency syndrome. Treatment was, until now, based on bone marrow transplantation. HLA identical bone marrow transplantation yields excellent results while those of HLA haploidentical bone marrow transplantation are not so good. A new therapeutic approach was developed recently, consisting of the intramuscular infusion of ADA enzyme covalently linked to polyethylene glycol (PEG-ADA). We report the results of this treatment in a 14 month-old child presenting with a partial form of ADA deficiency revealed by an opportunistic infection. This treatment corrected the immunodeficiency and the biochemical abnormalities as well. PEG-ADA infusions were well tolerated. The onset of an immunization against the ADA enzyme led to a drop in immunologic functions, which could be partially overcome by more frequent (biweekly) administration of the product. After a 18 month-follow-up the child is doing well, living normally at home. PEG-ADA represents a possible alternative for children presenting with ADA deficiency without any available HLA identical donor.