Treatment of hairy cell leukemia with 2-chlorodeoxyadenosine (2-CdA) induces in 85% patients complete remission. Complete remission is defined as the condition when signs of activity of the disease are absent, splenomegaly and lymphadenopathy are absent, the hemoglobin concentration is > or = 120 g/l, the absolute number of granulocytes is > or = 1.5 x 10(9)/l and the number of thrombocytes is > or = 100 x 10(9)/l. In complete remission in the peripheral blood, bone marrow aspirate and bioptic samples obtained by trephin bone marrow core biopsy, using standard staining (hematoxylin-eosin and May-Grünwald-Giemsa's method), no leukemic cells are present. When more sensitive methods are used (immunophenotyping, immunohistochemistry or molecular genetic methods), a persisting leukemic population can be detected which is described as minimal residual disease (MRD). For detection of MRD the authors used immunohistochemical examination of bone marrow with DBA.44 antibodies. As leukemic cells they described those which produced intense cytoplasmic and membrane positivity with antibody DBA.44 and corresponded morphologically to hairy cells. For evaluation computer analysis of the picture LUCIA-M was used. The infiltration grade was examined on three areas of standard size (3 x 65,265 micron 2) and expressed in percent. A total of 45 trepanobioptic specimens from 21 patients were examined who achieved after treatment with 2-CdA complete remission. In all samples suitable for evaluation the presence of leukemic cells (MRD) was detected with a median of 3% and a range of 1% to 18%. With induction of complete remission correlates also the low serum level of the soluble receptor for IL-2 (sIL-2R). In a female patient after 24 months of treatment with 2-CdA the grade of leukemic infiltration rose from 1% to 12% and during the 36th month to 50% DBA.44+ leukemic cells. The incipient relapse in this patient was not associated, despite marked infiltration of bone marrow, with failure of hematopoiesis and a marked rise of sIL-2R.