Report on a now 58-year-old female patient who presented in 1969 with the following objective findings: anemia, thrombocytopenia, lymphocytosis and hepatosplenomegaly. The first first diagnosis was lymphosarcoma, but up to 1976 nearly ten other diagnoses were established, e.g. reticulum sarcoma, leukemic lymphosarcoma, CML, ALL, CLL and Waldenström's disease. The patient did not respond to combined chemotherapy, but splenectomy brought about significant improvement in anemia and thrombocytopenia. She is now doing well without cytotoxic treatment. Hairy cell leukemia was diagnosed on the basis of electron microscopic findings in peripheral lymphocytes and histologic findings in bone marrow and spleen. The tartrate-resistant acid phosphatase was positive. Immunologic tests in the hairy cells showed surface immunoglobulins of monoclonal origina and the T-cells were shown to be decreased. The tests for phagocytosis and nonspecific esterase were negative. The hairy cells in this patient exhibited no colony stimulating activity in culture studies with bone marrow from 4 normal donors, as compared with the stimulating activity of normal monocytes in the same assay. In the light of this case report the question is discussed whether the hairy cells are mainly cells with lymphocytic (B-lymphocytes) or monocytic characteristics. On this question the findings in our patient support the hypothesis of a B-cell nature for the hairy cells.