Abstract Fourteen patients with hemopoietic and solid malignancies underwent dose-intensive chemotherapy, followed by haemopoietic stem cell grafting. Granulocyte apoptosis was assessed as nuclear chromatin condensation, after ex vivo incubation of blood samples for 3 h, followed by Acridine Orange staining. Myeloablative therapy caused time-dependent increase in ex vivo granulocyte apoptosis, being maximal at D+7 to D+9, thus preceding the development of neutropenia (nadir values on D+13 to D+15). In vivo administration of G-CSF (Neupogen) resulted into short-term decrease in apoptotic granulocyte numbers. Hence, granulocyte apoptosis is an important factor of therapy-induced leukopenia and could be modified by the G-CSF treatment.