The data on hairy-cell leukaemia (HCL) and resembling disorders in the literature and in our patients were analyzed to determine which clinical features and laboratory data are important for the recognition of HCL in an early stage. In pancytopenic patients the typical pattern of bone marrow involvement in HCL and the low number of monocytes in the peripheral blood appear to be essential for the differential diagnosis. In patients with many neoplastic cells in the peripheral blood, the presence of neutropenia and monocytopenia as well as tartrate-resistant acid phosphatase activity in the neoplastic cells, appears to be crucial for early diagnosis. Thus, the clinical features and routine laboratory data alone are sufficient in the majority of cases to suggest the diagnosis HCL. The monocytopenia proved to be most helpful in this respect. Nevertheless, in all patients, and certainly in patients presenting with atypical features, a bone marrow biopsy is indispensable for the correct diagnosis.