Decreased leukocyte values in children are usually due to the decrease in the number of neutrophilic granulocytes. This is usually a transient phenomenon associated with viral infections. In infancy and early childhood, immune mediated neutropenias are possible causes of prolonged leukopenia. Causes of rare leukopenias are numerous, including underlying diseases such as congenital myelopathy, a syndrome or malignant hematological disease. The risk of infection associated with neutropenia is increased especially in patients with a production defect of the bone marrow as the underlying cause.