The immunophenotypic analysis of ex vivo-expanded mesenchymal stromal cells (MSC) has so far been confined to single or dual staining analysis in normal subjects. In this study, using a four-color cytofluorimetric protocol, we demonstrated that cultured MSC derived from the bone marrow of patients with hematologic malignancies showed alterations in the expression of CD105, CD90, CD184, and HLA-DR molecules. The decrease in the percentage of CD105+ and CD90+ MSC correlated with an increased bone marrow angiogenesis. This paper provides evidence that multiparametric flow cytometry is essential for the establishment of a standardized protocol to identify various MSCs subsets and aberrant phenotypes.