Congenital immunodeficiency is one or combined immune defect in immunoglobulin, leukocyte, and complement. These patients have increased susceptibility to respiratory infection. Hence, their infection must be taken care of, tried to gene therapy and stem cell transplantation. We present here a case of hyper-IgM syndrome in an 11-year-old male patient who complained of abdominal distension and abdominal pain. Multiple abdominal masses were detected by abdominal computed tomography (CT) and he was diagnosed with neuroendocrine carcinoma by mass biopsy. There was no evidence of metastasis of cancer cells to the bone marrow, but a dysgranulopoietic feature was noted and he was diagnosed with childhood myelodysplastic syndrome. This is the first report that neuroendocrine carcinoma is associated with childhood myelodysplastic syndrome in hyper-IgM syndrome. (Korean J Hematol 2009;44:330-335.)

Keywords: Hyper-IgM syndrome, Neuroendocrine carcinoma, Childhood myelodysplastic syndrome