We report a 3 1/2-week-old male infant with Down's syndrome who presented with abdominal distention, ascites, and eosinophilia and was subsequently diagnosed as having an abdominal non-Hodgkin's lymphoma of T-cell type. Because of the unusual association of these two conditions, especially at this early age, the literature was reviewed regarding the possible oncogenic mechanisms in Down's syndrome patients and the various malignancies associated with this condition. Non-Hodgkin's lymphoma is discussed briefly with emphasis on its possible etiologic mechanisms and predisposing conditions, especially the immunodeficiency states. Because this infant presented with non-Hodgkin's lymphoma at an early age, it is considered unlikely that an immunoaberration is responsible. Also, a short discussion of this patient's peripheral and bone marrow eosinophilia is given, implicating a T-cell product (e.g., eosinophil differentiation factor) as the putative pathophysiologic mechanism.