The authors describe a patient who presented an association of hairy cell leukemia (HCL) and large granular lymphocyte (LGL) leukemia. An eventual relationship between these two rare entities is analyzed. Hairy cells (HCs) were present in the blood, bone marrow, and spleen. An excess of LGLs was found only in the blood and bone marrow. After splenectomy the patient received an alpha 2-interferon (alpha 2-IFN) treatment. The HCs surface phenotype was mu+delta+kappa+, CD20+, and CD25+. The LGLs consisted in CD3+, CD8+, HNK1+, WT31+ T lymphocytes. These were absent in the spleen. alpha 2-IFN treatment resulted in the disappearance of the HCs in the blood and bone marrow, whereas the LGLs remained unchanged. Before alpha 2-IFN treatment, peripheral blood cells, predominantly LGLs, exerted low cytotoxicity that increased up to a normal level after treatment. Using Southern blotting the authors studied the rearrangements of the T-cell receptor beta--chain (C beta) and gamma-chain (J gamma) genes and immunoglobulin heavy (JH)- and light (C kappa, C lambda)- chain genes. An unique JH and C kappa gene rearrangement was found in the blood and spleen, whereas C beta and J gamma gene rearrangements were present in the blood, not in the spleen. Under alpha 2-IFN treatment, the JH gene rearrangement fainted dramatically, in contrast to that of the C beta gene. The study of messenger RNA (mRNA) of the T cell receptor alpha and beta chains evidenced the 1.3-kilobase (kb) and 1.6-kb bands in the blood and their absence in the spleen. The patient was human T-cell leukemia virus (HTLV)-II negative by Southern analysis of blood and spleen cells. It is concluded that the LGL expansion was clonal and not reactive to the HCL. Although the authors cannot definitely exclude that both HC and LGL proliferations stem in a common leukemic precursor, their findings support an association of the two entities.