Lymphoproliferative disorder (LPD) with polymorphous composition of proliferation (polymorphous LPD), containing large lymphoid cells together with small lymphocytes, plasma cells, macrophages, and/or eosinophils, is found in individuals with immunodeficiency conditions. Clinicopathological findings in 19 cases of polymorphous LPD registered with the Osaka Lymphoma Study Group, Osaka, Japan, were analyzed; they represented 0.4% of the registered cases. In six cases, there was a history of rheumatoid arthritis; five of them had received immunosuppressive agents. There were no acquired immunodeficiency syndrome cases or organ transplant recipients. Southern blotting and/or polymerase chain reaction (PCR)-based clonality analysis revealed monoclonal B cell and T cell proliferation in eight and six cases (B- and T-LPD), respectively, and polyclonality in one. In B-LPD, there was polymorphous proliferation, containing large B-lymphoid cells, while medium-to-large T lymphoid cells with occasional eosinophilic infiltration were seen in T-LPD. Epstein-Barr virus (EBV) was detected in three of eight B-LPD, four of six T-LPD, and one of one polyclonal LPD. The prognosis was not favorable; the 3-year overall survival rate was 49.7 +/- 17.3%. Thus, polymorphous LPD is relatively rare in Japan and is a heterogeneous disease with monoclonal proliferation of B or T cells; additionally, it is occasionally EBV-associated, and behaves as an aggressive lymphoma.