International Extranodal Lymphoma Study Group (IELSG) is a unique cooperative group. The idea of setting up an international group of investigators to study the extranodal lymphomas on Janruary 23-24, 1998 during a mid-summer meeting of the lymphoma group at the Oncology Institute of Southern Switzerland IOSI.

By bringing together numerous scientists from different institutions, it has become possible to amass the data from a sufficient number of patients to study specific extranodal sites of involvement by a lymphoma.

Although extranodal lymphomas are not rare, the frequency of involvement of any particular site is not high enough for a single institution to answer the major questions. IELSG should be able to accomplish this. It has an excellent group of worldwide experts in lymphomas as leaders. It has also brought together a number of outstanding institutions in the fight against extranodal lymphomas.

IELSG became full UICC member in 2005.