In acute lymphoblastic leukemia (ALL) the apoptosis of blast cells in peripheral blood (PB) and bone marrow before and/or during treatment, is of great interest. As the morphological changes during apoptosis provide the most reliable markers, in the present study we utilized a nuclear stain based on ethidium bromide (EtBr) for the rapid qualitative and quantitative measurement of circulating apoptotic cells directly in PB suspensions without fractionation. By using a fluorescent microscope the apoptotic cells appeared clearly visible, making the estimation of their percentage straightforward. We studied apoptosis before and during the onset of chemotherapy in PB from 16 children with ALL at diagnosis, and one upon relapse. In the cases studied at diagnosis the circulating apoptotic cells were found in variable percentages after 24 hours of treatment. Maximal apoptosis was observed after 24 hours of treatment in five cases and after 48 hours in two cases. After 96 hours of treatment the cases studied at diagnosis could be divided into three groups: those with a) negligible apoptotic cells, b) between 8% and 12% apoptotic cells and c) a high percentage of apoptotic cells (more than 20%). The relapsed case was characterized by P-glycoprotein positive blast cells, and circulating apoptotic cells which remained very low at all time points. Thus, it is possible to evaluate the response to treatment by studying apoptosis directly in peripheral blood. Therefore, the maximum apoptotic effect and the percentage of circulating apoptotic cells at the different time intervals must be considered.