Chronic B-lymphocytic leukemia (B-CLL) is one of the most frequent diseases of the lymphatic system in adults. It is known for its variable course--from benign to prospectless forms. Reliable prognostic markers are needed to precify the diagnosis and the correct therapy. The ZAP-70 protein, as a prospective marker, can be highly expressed in leukemic cells of B-CLL patients. It is related to a worse prognosis requiring intense therapy from the beginning. The present study includes 20 cases of bone marrow trephine biopsy from patients with diagnosed B-CLL and 20 control cases without neoplastic infiltration. The specimens were investigated with standard immunohistochemical technique and with the use of the amplification system (DakoCytomation). The results were evaluated semiquantitatively as negative (less than 5%), positive (above 30%), and irregularly positive (5-30% of positive leukemic cells) and consecutively correlated with clinical evaluation of prognosis of the patient. The evaluation of the positivity was controlled also by morphometric analysis by determination of the area of ZAP-70 positivity related to the whole area of nuclei in the section. In the control specimens the ZAP-70 positivity was restricted to T-lymphocytes only. The level of the nuclear positivity of ZAP-70 protein detected in transformed B-lymphocytes showed significant correlation with the worse or the better clinical prognosis, respectively. Cases with irregular positivity did not show unambiguous clinical correlation. The use of the amplification system allowed to apply lower concentration of the primary antibody, reduction of background staining and increased the contrast of the findings leading to reduction of irregularly positive cases from 30% (with routine histochemistry) to 10%. It can be concluded that ZAP-70 represents a valuable prognostic marker for the chronic B-lymphocytic leukemia. Its evaluation by histochemistry is a suitable method for clinical praxis. Problematic may be the evaluation of borderline irregularly positive cases, in which the use of the histochemical amplification system is helpful. Strictly determined criteria are needed for limitation of inaccurate interpretation of the results.