The first step in evaluating leukopenia is the analysis of the different leukocyte subpopulations. The automated total blood cell count gives a first impression of the decreased leukocyte subtype and if erythrocytes and/or platelets are involved. Microscopic interpretation of the blood smear verifies the automated differential and allows a statement on the morphology of the individual cells. Differential diagnosis of the decreased leukocyte subpopulation is vast and in many cases leukopenia is only an epiphenomenona of a systemic disease. Therefore therapy is always directed towards the underlying disorder.