Summary. Peripheral blood stem cell (PBSC) mobilization using idarubicin and cytarabine was investigated in 40 patients with chronic myeloid leukaemia in first chronic phase (CML CP1). Disease contamination was evaluated in harvests from 41/44 (93%) mobilization episodes. Using cytogenetics, 22/37 (59%) showed a complete or major response; Southern blot analysis demonstrated a complete or major response in 9/17 (53%). No harvests were RT-PCR negative. In the 41 evaluable episodes, more complete or major responses were seen when PBSC mobilization occurred within 24 months [17/23 (74%) versus 6/18 (33%); P ¼ 0 : 02] and within 12 months of diagnosis [10/11 (91%) versus 13/30 (43%); P ¼ 0 : 018]. 20 patients underwent PBSC transplantation and 18/20 successfully engrafted. Post-transplant cytogenetic analysis was available on 15 cases, of whom five achieved a major cytogenetic response at 1-3 months, with five partial cytogenetic remissions. Two of 40 patients died during mobilization therapy (5%) and three of 20 after the transplant (15%). Overall mortality was high at five of 40 patients, and the procedural mortality was 20%. This study demonstrates that Ph-negative PBSCs can be mobilized in a significant proportion of patients with CML CP1, with the best results observed within a year of diagnosis. These cells can subsequently be used for autologous transplantation, however, the impact on long-term survival requires longer follow-up, and potential benefits may be compromised by the high mortality.