We have previously reported a more than 50% risk of insufficient harvests (< 2 x 10(6) CD34+ cells/kg) following a fixed timing of leukapheresis in an unselected patient population collected for autologous stem cell transplantation. The failures were easily identified as patients who had < 20,000/ml CD34+ blood cells at the time of harvest. Consequently, in an attempt to optimize the timing strategy, we decided to perform daily analysis of the blood levels and perform leukapheresis within 24 h after the blood cell count exceeded 20,000 CD34+/ml. The results of this prospective study showed (a) that leukapheresis collections that were initiated based on the CD34+ blood level resulted in a success rate of 85% following 1-3 day harvests, (b) that high-level mobilizers with > 80,000 CD34+ cells/ml can be harvested successfully by only one procedure, and (c) that low-level mobilizers with < 40,000 CD34+ cells/ml still have a high risk of inadequate harvests. In summary, the optimized timing strategy is effective, but there is still room for improvement. First, new potent priming procedures for low-level and bad mobilizers are required, and second, algorithms to reduce the leukapheresis volume and time for high-level mobilizers should be established.