The present report describes two young males with clinically diagnosed infectious mononucleosis (IM) who subsequently were diagnosed as having malignant B-cell lymphoma (i.e., immunoblastic sarcoma of B-cells). Despite these apparent similarities, there were fundamental differences between the two cases. The first patient, whose lymphoma was diagnosed 9 months after IM, was one of a well-described kindred with the X-linked lymphoproliferative syndrome (XLP) in which affected young males lack the ability to mount an effective immune response to primary infection with the Epstein-Barr virus (EBV) (i.e., infectious mononucleosis), and subsequently develop fatal lymphoproliferative disorders of the B-cell type. This was in contrast to a second patient, also a young male, who did not have the X-linked lymphoproliferative syndrome, who did develop specific antibodies to the Epstein-Barr virus and whose malignant lymphoma was closely associated in time (i.e., 5 weeks) with the clinical diagnosis of infectious mononucleosis. The comparative immunologic and virologic features are discussed as well as the importance of careful clinicopathologic correlation in young adults and children developing malignant lymphoma both following and in association with infectious mononucleosis.