The Lymphoma Study Group (LSG) of the Japan Clinical Oncology Group (JCOG) was initiated in 1978 by five institutions and now has 47 members. JCOG-LSG has focused on combined modalities, dose intensification and the incorporation of new agents for major disease entities of lymphoid malignancies. More than 30 trials including 10 randomized trials have been conducted for aggressive non-Hodgkin's lymphoma (NHL), adult T-cell leukemia-lymphoma (ATL), lymphoblastic lymphoma/acute lymphoblastic leukemia, Hodgkin's lymphoma (HL), multiple myeloma, NK/T-NHL and indolent B-NHL, and correlative epidemiological and pathological studies have been performed on human T-lymphotropic virus type-I and T/B cell phenotypes. The first trials for aggressive NHL revealed significant differences in the prognosis of ATL, non-ATL T-NHLs and B-NHLs, establishing a subclassification of ATL, and leading to the establishment of standard therapies for ATL and localized nasal natural killer/T-NHL. Recently, for B-NHLs including diffuse large B-cell lymphoma, mantle cell lymphoma, and indolent B-NHLs, regimens incorporating rituximab have been evaluated. The JCOG-LSG trials for HL led to the approval of dacarbazine for the National Health Insurance in Japan. The multicenter trials by the JCOG-LSG combining new modalities such as molecular-targeting agents will contribute to further improvements in the treatment of lymphoid malignancies.