A 7.5-month old male presented with increasing respiratory distress progressing to severe hypoxia, an erythematous scaling rash and a paucity of lymphoid tissue. Chest x-ray revealed bilateral pneumonia with diffuse alveolar opacities. He had an increased neutrophil count, normal hemoglobin and platelet count, and an absolute lymphocyte count of 4.3 x 109/L decreasing to 1.8 x 109/L. IgG was 1.72 g/l, IgA 0.59 g/l and IgM 1.08 g/l, but no antibody to tetanus, diphtheria or pneumococcus despite immunization. Nasopharyngeal aspirate demonstrated rhinovirus by PCR, and tracheal aspirate was positive for Pneumocytis jiroveci by immunofluoresence. Lymphocyte markers showed: 62% CD19+, 24% CD4+, 1% CD8+ cells and 0.5% NK cells. CD4+ cells were 90% CD45RO+, 8% CD45RA+ and 5% were CD25+. His cells had no proliferative response to anti-CD3 or IL-2 stimulation, a weak response to PHA, and no response to antigen or MLR. BMT was performed from a HLA-identical sister and the patient is well 7 months post BMT.