The clinical manifestation, diagnosis, pathogenesis, pathophysiology and treatment of purine nucleoside phosphorylase (PNP) deficiency is overviewed. Among the severe combined immunodeficiency syndrome (SCID), PNP deficiency was 2ndly found as an autosomal recessively heritable purine metabolic disease. This disease is characterized by severe T-cell immunodysfunction accompanied by marked low level of uricemia and uricosuria. In addition, this concurrence strongly suggests the diagnosis of PNP deficiency. Neurological manifestations are found in two-third of the reported cases. About 30 cases have been reported worldwide. Their ages at onset varied from birth to 6 years and their outcome is usually fatal terminating in viral infection or lymphoma. Our first case and her sister died with varicella and lymphoma, respectively. No clinical benefit was found with normalized uricemia after erythrocyte transfusion in our case. Bone marrow transplantation and future gene therapy are reviewed.