Specific application of immunological markers in acute lymphoblastic leukemia (ALL) for detection of inadequate blast cell reduction after induction therapy or early recognition of a. relapse requires a precise characterization of the immunophenotype at the time of diagnosis and an understanding of the biology of the disease. Therefore, the ALL-BFM 83 study is first used to demonstrate the incidence, antigen expression and dynamics of relapse occurrence of immunological subtypes in childhood ALL. This is then followed by a discussion of the different immunological features hitherto applied for identification of residual leukemia cells in ALL (terminal deoxynucleotidyl transferase – TdT; common acute lymphoblastic leukemia-associated antigen – CALLA, CD10; various T-cell differentiation antigens; kappa/lambda labelling); these are, however, not leukemia-specific and are expressed to varying degrees by normal lymphoid progenitor cells. The sensitivity of these analyses is therefore largely determined by the markers applied and the type of investigational material. Finally, suitable markers are presented for detecting residual leukemia cells in the different immunological subtypes of ALL. Their clinical relevance still remains to be evaluated in prospective therapy stud