We observed two cases of Clostridium tertium bacteremia three months apart in the sterile unit of our department of hematology and oncology. One patient was being treated for first-relapse acute myeloblastic leukemia, while the second was receiving high-dose chemotherapy with hematopoietic stem cell support for non-Hodgkin lymphoma. At the time that C. tertium was identified, the first patient was completely asymptomatic, while the second was highly febrile. Both responded biologically and/or clinically to antibiotherapy. We discuss the epidemiology and pathology of C. tertium in the general and cancer patient population.