The current study was undertaken to compare biopsy and aspiration sampling techniques for reliable cell kinetic analysis in acute leukemia and to reevaluate the clinical significance of cell kinetic data in this disease. Ad 1: One hundred forty-three paired simultaneously taken marrow aspirates and Jamshidi biopsies from 61 patients with acute nonlymphocytic leukemia (ANLL) were compared by means of flow cytometry (FCM) for cellular DNA and RNA measurements, autoradiography for 3H-TdR labeling index and liquid scintillation counting for 3H-TdR uptake. Aspirates were found unreliable for pretreatment evaluation and for monitoring of drug induced cell kinetic perturbations due to contamination with peripheral blood. Aspiration should, therefore, be replaced by marrow biopsy which provides reliable sampling of bone marrow for cell kinetic studies. Ad 2: FCM S-phase index determined from biopsy material was not found predictive for therapeutic response in 43 patients with ANLL treated with two different induction regimens. In one of the two protocols, changes in FCM S-phase index after 48 hours continuous infusion of cytosine arabinoside correlated significantly with clinical response and nonresponse, respectively.