We have presented a case of diffuse mixed lymphocytic and histiocytic lymphoma that appeared as persistent oral ulcerations in a pituitary dwarf with combined variable immunodeficiency. The difficulty in diagnosis is emphasized and the differential features of polymorphic reticulosis, Wegener's granulomatosis, malignant lymphoma, and idiopathic midline destructive disease have been reviewed. Awareness of these disease processes and their distinguishing features may expedite accurate diagnosis.