Flow cytometric enumeration of CD34+ hematopoietic stem and progenitor cells (HSCs) is widely used for evaluation of graft adequacy of peripheral blood and bone marrow stem cell grafts. In the present study, we review and compare the major counting techniques of stem and progenitor cells. The methods are: the Milan/Mullhouse protocol, two-platform ISHAGE (International Society of Hematotherapy and Graft Engineering) and single-platform ISHAGE analysis system. According to the Milan/Mulhouse protocol, HSCs are identified by CD34 antibody staining and easy gating strategy. The ISHAGE guidelines for detection of CD34+ cells are based on a four-parameter flow cytometry method (CD34PE/CD45PerCP staining, side and forward angle light scatter) thus employing multiparameter gating strategy. With two-platform ISHAGE protocol, an absolute CD34+ count is generated by incorporating the leukocyte count from an automated hematology analyser. The single-platform ISHAGE method to determine the absolute CD34+ count directly from a flow cytometer includes the use of Trucount tubes (Becton Dickinson) with a known number of fluorescent beads. CD34+ cells were quantified in mobilized peripheral blood, collected by leukapheresis, and bone marrow from 42 samples from patients with hematological malignancies. The differences against the means display low disagreement between the Milan/Mulhouse and ISHAGE protocols, with discrepancies of up to 2.5% (two-platform ISHAGE)--2.6% (single-platform ISHAGE) in enumeration of CD34+ cells in leukapheresis product and 4.8% (two-platform ISHAGE)--4.9% (single-platform ISHAGE) in bone marrow. Our results show high correlation among all three methods. Since the three protocols are compatible, choosing the most convenient in terms of costs, simplicity and compliance with clinical results appears to be a logical consequence.