The bone marrow histological picture was studied in 13 patients with hairy-cell leukemia (HCL) who were followed up at the Hematological Centre in the Latvian SSR. The diagnosis was made or proved in all the cases, among them in 9 patients - on the basis of the proliferation type of leukemia cells in the bone marrow trephine biopsy that is characteristic of HCL. The analysis of the clinical and morphological features of the disease has permitted the authors to distinguish specific types of HCL: 1) splenic and bone-marrow variants predominantly were diagnosed clinically, 2) typical and atypical variants resembling malignant lymphoma or chronic lymphoid leukemia were diagnosed histomorphologically. The authors have recommended trephine biopsy+ for a wider use (as a more informative method as compared to sternal punction) in the diagnosis and differential diagnosis of HCL.