Background aims To obtain a cell product competent for clinical use in terms of cell dose and biologic properties, bone marrow-derived mesenchymal stem cells (MSCs) must be expanded ex vivo . Methods A retrospective analysis was performed of records of 76 autologous MSC products used in phase I or II clinical studies performed in a cohort of cardiovascular patients. In all cases, native MSCs present in patient bone marrow aspirates were separated and expanded ex vivo . Results The cell products were classified in two groups (A and B), according to biologic properties and expansion time ( ex vivo passages) to reach the protocol-established cell dose. In group A, the population of adherent cells obtained during the expansion period (2 ± 1 passages) was composed entirely of MSCs and met the requirements of cell number and biologic features as established in the respective clinical protocol. In group B, in addition to MSCs, we observed during expansion a high proportion of ancillary cells, characterized as osteoclast precursor cells. In this case, although the biologic properties of the resulting MSC product were not affected, the yield of MSCs was significantly lower. The expansion cycles had to be increased (3 ± 1 passages). Conclusions These results suggest that the presence of osteoclast precursor cells in bone marrow aspirates may impose a limit for the proper clinical use of ex vivo expanded autologous bone marrow-derived MSCs.