The success of peripheral blood progenitor cell (PBPC) transplantation depends upon harvesting adequate numbers of cells and accurate prediction of when to commence apheresis. Although peripheral white cell count (WBC) is commonly used to identify when to initiate apheresis it does not uniformly predict the CD34+ content of the apheresis product nor the number of exchange procedures required. We investigated whether the peripheral blood CD34+ count would not only predict harvest yield but whether it would also predict the number of apheresis procedures needed to generate at least 2 x 10(6)/kg CD34+ cells. CD34+ counts were performed over an 8-month period on the peripheral blood and PBPC harvests of all patients undergoing leucopheresis. Regression analysis showed a highly significant correlation between peripheral blood CD34+ count and yield of CD34+ cells in the apheresis product. The regression plot with WBC was weaker. We have shown that a peripheral CD34+ count > or = 62 x 10(6)/l is required to confidently achieve an adequate harvest in one apheresis, two aphereses are needed if the initial count is > or = 40 x 10(6)/l. Therefore peripheral blood CD34+ counts not only are able to determine the threshold at which to commence apheresis but are useful in predicting apheresis requirements and planning demands on the apheresis service.