Pre transplant screening work-up of donors for allogeneic blood and marrow transplantation is essential in an effort to minimize risks to the recipient and protect the donor. At Princess Margaret Hospital, every potential donor is screened with a bone marrow aspirate. The case histories of three asymptomatic potential donors who presented within 1 year with normal complete blood counts, history and physical examination are presented. A 65-year-old male patient was diagnosed with smouldering multiple myeloma, a 72-year-old male patient with chronic lymphocytic leukemia and a 42-year-old male patient with myelodysplastic syndrome. Bone marrow examination led to the diagnosis in each one of these cases. Of note is that each of the potential donors was discovered to have the same disease as the transplant recipient. In vitro clonogenic hemopoietic progenitor assays were compared to those of 20 normal volunteers. Inferior growth of hemopoietic progenitor colonies in all three was noted. In conclusion, particularly in older donors and donors with potential for familial malignancies, more screening investigations including bone marrow aspiration may be reasonable to investigate for occult hematological malignancies prior to stem cell donation. Clonogenic assays can contribute to detect hemopoietic abnormalities pre transplant.