BACKGROUND: The yield of CD34-positive cells obtained from an apheresis procedure is determined, in part, by the efficiency of collection. Optimization of the efficiency of CD34-positive peripheral blood cell collection requires identification of predictive factors. STUDY DESIGN AND METHODS: Demographic, stem cell collection, mobilization, and disease-related measures from autologous and allogeneic donors undergoing 252 progenitor cell apheresis procedures were retrospectively reviewed. Statistical relationships between CD34 collection efficiency and the various measures were determined by correlation and multiple linear regression analysis. RESULTS: CD34 collection efficiency inversely correlated with the peripheral white cell count, hematocrit, and serum albumin concentration (R2 = 0.29). White cell count was the single best predictor of CD34 efficiency (R2 = 0.19). Donor groups with cytopenias (patients vs. normal donors; increased cycles of prior chemotherapy; bone marrow involvement; chemotherapy plus growth factor mobilization) had higher collection efficiencies. Only 29 percent of the variability in the data could be attributed to white cell count, hematocrit, and albumin concentration. The majority of the remaining variability was due to unexplained differences between donors. CONCLUSION: CD34 collection efficiencies show considerable variation. Higher peripheral white cell counts, hematocrits, and/or albumin concentrations result in decreased CD34 collection efficiency, but most of the variability in the data is not accounted for by these three factors.