In chronic lymphocytic leukaemia (CLL) bone marrow trephine biopsy (BMT) is not required for diagnosis but can have a significant prognostic value and can be used for the detection of the minimal residual disease (MRD) and for assessment of the effectiveness of the treatment applied. The aim of the study was to evaluate the morphological changes in bone marrow after treatment with purine nucleoside analogues cladribine and fludarabine. Bone marrow trephine biopsy was taken routinely from 15 patients with CLL. Bone marrow trephine biopsy was performed on every patient before as well as after chemotherapy. The number of cell elements of the marrow (the degree of atrophy), the patterns of bone marrow infiltration, the presence of reticulin and collagen fibres and the disturbances in bone marrow stroma were assessed. The infiltration of bone marrow by neoplastic cells was observed in all the patients before administration of chemotherapy. The infiltration was followed by an increase in the number of reticulin fibres. After the treatment a regression of the reticulin fibres was observed with the lessening of the infiltration. After the treatment the levels of marrow infiltrate were decreased. Increased hypoplasia of the bone marrow was observed after the chemotherapy.