We studied the utility of G-CSF for harvesting circulating haematopoietic stem cells in patients with leukaemia or lymphoma based on a comparative study in a single patient. Two successive cycles of leukapheresis following cytotoxic chemotherapy were performed in 22 patients as follows: the first cycle was performed with cytotoxic mobilization in all patients while the second cycle was randomized into two groups: cytotoxic (n = 10) and cytotoxic plus G-CSF (cytotoxic/G-CSF) (n = 12) mobilization. Repetitive cytotoxic mobilization did not alter the yields of mononuclear cells (MNC), myeloid (CFU-GM), and erythroid (BFU-E) progenitors. In contrast, cytotoxic/G-CSF mobilization produced significantly higher yields of MNC (2.6-fold), CFU-GM (5.5-fold), and BFU-E (3.9-fold) than did cytotoxic mobilization alone (P < 0.01). The ratio of CFU-GM to BFU-E was not affected by G-CSF. Furthermore, G-CSF led to an earlier peak of CFU-GM following chemotherapy. G-CSF is thus effective in expanding the pool of circulating haematopoietic progenitors.