Purine nucleoside phosphorylase (PNP) deficiency is a rare immunodeficiency syndrome generally characterized by profound T cell deficiency and variable B cell function. More than half of PNP-deficient patients present with neurological dysfunction, with manifestations such as mental and motor retardation, spasticity, hypertonia, ataxia, and behavioral disturbances. Here, we report two siblings diagnosed with PNP deficiency in early infancy. Our patients had developmental delays, and their immunological findings indicated T-B+NK+ leaky/atypical severe combined immune deficiency. The patients are being treated with regular intravenous immunoglobulin replacement, as well as trimethoprim-sulfomethoxazole and fluconazole, for prophylaxis in preparation for transplantation. These cases draw attention to the possibility of primary immune deficiency in patients with recurrent infections and lymphopenia. In addition, PNP deficiency should be kept in mind in the presence of developmental delay, low uric acid levels, and lymphopenia.