A 16-month-old female infant presenting with pancytopenia and fever was found to have pulmonary aspergillosis and large-cell immunoblastic non-Hodgkin's lymphoma of peripheral post-thymic origin isolated to bone marrow. Extensive noninvasive evaluations failed to demonstrate the disease in other extramedullary sites. The malignant cells were large and polymorphous; lacked terminal transferase; expressed surface CD-2, CD-3, CD-8, and HLA-DR antigens, and showed rearrangements of the T-cell-receptor beta-chain gene. To our knowledge, this type of lymphoma in an infant has not been reported before. Furthermore, aspergillosis is a rare presenting feature in patients with lymphoproliferative disease. In our case, it may reflect an underlying immune deficiency associated with the transformation and proliferation of a suppressor T cell.