A female patient, at the moment 2½ years old, with a purine nucleoside phosphorylase (PNP) deficiency showed a gradual decrease of T cell immunity during the first year of life whereas B cell immunity developed normally (1) Later on in life some disturbance of B cell immunity also developed as could be concluded from the appearance of homogeneous immunoglobulins in the serum from the decrease in the synthesis of specific antibodies. Earlier studies (1, 2) have shown that an inherited PNP deficiency existed in two older sisters of the patient, both suffered from the same immunodeficiency and have died. The oldest sister has been treated with several thymus transplants, resulting in a partial and transient restoration of T cell function. However the most obvious therapeutic approach of these patients would be correction of the consequences of the metabolic defect. Therefore, the patient under study was treated by oral substitution with uridine and purines and afterwards with several transfusions of irradiated red cells and plasma. The former treatment was rather unsuccessfull, the latter resulted in a remarkable change in the metabolic state of the patient and some improvement of in vicro immune functions. In contrast to findings of Polmar et al. (3) in a ADA deficient patient, complete immunological restoration could not be Achieved by this treatment until now.