After morphology and cytochemistry, immunological leukemic cell diagnosis became a generally available method by the development of monoclonal antibodies. Besides an otherwise not possible classification of nonmyeloid leukemias according to T- or B-differentiation status, precision and standardization of leukemia diagnosis is accomplished by immunodiagnostic means in an experimental way, an important point for therapy studies. In children, the common ALL - a precursor B-ALL type - was prognostically more favorable than the T-ALL which differs also in clinical appearance and in prognostic factors. In adults, however, T-ALL had a better prognosis than common ALL in a prospective trial. The heterogenous Null-ALL which makes up 23% of ALL in adults and 5% in children, was the prognostically most unfavorable form besides the rare B-ALL (1-2%), the immunophenotype being an independent prognostic variable. Immunodiagnostic means are therefore of utmost clinical importance compared to other methods such as cytogenetic and electron microscopy that are not performable in a corresponding extent.