[Incidental finding lymphocytosis. From normal variants up to leukemia].

A relative or absolute increase of lymphocytes in peripheral blood is a frequent incidental finding. The differential diagnosis of such a finding includes a broad spectrum ranging from normal variations to neoplastic diseases. As for other laboratory parameters lymphocytosis must be interpreted within a given clinical situation. Further diagnostics aim to reliably differentiate between reactive and neoplastic conditions. If a lymphoma or leukemia is suspected this should lead to a rapid hemato-oncological investigation.