We studied the peripheral blood of 37 patients with hairy cell leukemia (HCL) prior to (n = 24) or following (n = 19) splenectomy, in the Hemalog D multi-channel white cell differential counter, to investigate whether the apparatus could contribute to the (early) diagnosis of this entity and to the differential diagnosis of HCL from atypical hairy cell leukemia (AHCL; n = 9), chronic lymphocytic leukemia (CLL; n = 21) and leukemic non-Hodgkin lymphoma of low-grade malignancy (LNHL; n = 19). HCL showed almost invariably monocytopenia, neutropenia and an increased percentage of LUC, with a rather typical picture of the X-Y display of the peroxidase channel. The percentage of hairy cells closely correlated with the percentage of the LUC from the Hemalog D. Discriminant analysis using several parameters of the Hemalog D differential count resulted in a complete separation of HCL from CLL, and a fair, although not complete, distinction of HCL from AHCL and LNHL. It was impossible to discriminate between AHCL and LNHL. The most important discriminating (single or combined) parameters were the absolute monocyte count, the TWBC and the absolute neutrophil number. It is concluded that the Hemalog D is a valuable tool in the (early) diagnosis of HCL and in the discrimination between HCL and other leukemic lymphoproliferative disorders.