Apoptosis, programmed cell death, occurs in a variety of cellular systems and in response to many different stimuli. One group of apoptosis inducers are glucocorticosteroids which are also found in the battery of cytotoxic drugs used to treat CLL. In the present study we have examined the potency of the glucocorticosteroid-dexamethasone to induce apoptosis in lymphocytes of patients with B-CLL. Lymphocytes of 15 nontreated patients and 5 controls were isolated and incubated for 24 h in the presence or absence of dexamethasone (2 mu M) Following incubation the cells were harvested and their DNA extracted. The extracted DNA samples were analysed for internucleosomal DNA cleavage by UV illumination after electrophoresis on agarose slab gel containing ethidium bromide. Five patients showed neither spontaneous nor dexamethasone induced apoptosis. Whereas, 10 patients, showed a dexamethasone-non-dependent spontaneous apoptosis which appeared 24 h after the start of incubation. The cells of these patients were the only ones to respond to dexamethasone showing an enhanced apoptosis effect. This study shows that apoptosis monitoring in CLL may provide important information regarding susceptibility of the cells to steroid induced apoptosis.