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 concentrati- on is > 120 g/l, the absolute number of granulocytes is > 1.5 x 10 9 /l and the number of thrombocytes is > 100x10 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-eo- sin 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 mar- row with DBA.44 antibodies.As leukemic cells they described those which produced intense cyto- plasmic 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 65265 mm 2 ) and expressed in percent. A total of 45 trepanobioptic specimens from 21 patients were examined who achieved after treat- ment with 2-CdA complete remission. In all samples suitable for evaluation the presence of leuke- mic 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 with2-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.

        Key words: Hairy cell leukemia - Minimal residual disease - DBA.44 - Soluble interleukin 2