Sequential bone marrow aspirates obtained from 10 children with relapsed acute nonlymphocytic leukemia (ANLL) after a high dose of cytosine arabinoside (Ara-C; 1000 mg/sq m) were analyzed by flow cytophotometry. The drug causing elimination of proliferating cells followed by a synchronous wave of cell recruitment. Among individual patients, considerable variation was observed in the degree of recruitment as well as in the time of appearance of the recruitment maximum (range 17–36 hr). However, both parameters appeared inversely correlated with the proliferative status in the bone marrow before treatment. In 6 other patients, cell kinetic responses were studied during treatment with repeated Ara-C injections scheduled individually according to the expected optima of recruitment. Waves of recruitment could be observed during 4–5 consecutive injections. The results suggest that in childhood ANLL, characteristic and individual cytokinetic responses to treatment with high-dose Ara-C can be monitored during therapy. These observations may allow the development of individual treatment schedules.