Polymorphic B cell lymphoma and diffuse B cell lymphoproliferation associated with Epstein-Barr virus infection is increasingly reported in immunodeficient patients. Accurate diagnosis of these pathologies is essential because the appropriate treatment regimens for the patients in question differ from those for patients with other lymphoproliferative diseases. Two complementary techniques are currently used in the diagnosis and characterization of Epstein-Barr virus-associated B cell lymphomas and diffuse B cell lymphoproliferation. Immunofluorescence allows specific detection of Epstein-Barr nuclear antigens in lymphomatous tissue. Molecular hybridization with the Bam H1-W and/or Bam H1-NJ probes confirms the presence of the Epstein-Barr virus genome in tumour cells. The Bam H1-NJ probe is also useful in determining the clonality of the tumour and the replication mode, episomal or linear, of the viral genome. The polymerase chain reaction method allows detection of the Epstein-Barr virus genome within 24 h in these tumours and is more sensitive.