Three chemotherapy-induced neutropenic hematologic patients with severe systemic infection caused by Fusobacterium nucleatum, a Gram-negative anaerobic rod, are described. Anaerobic infections are not very common in this patient category, but in a short period of time, several such patients were seen. The infection was considered to be caused by a combination of chemotherapy-induced mucositis, which served as a portal of entry for the systemic infection, and the antibiotic regime used in these patients. This is a serious infection with a high mortality. In hematological neutropenic patients suffering from severe mucositis and fever, antibiotic therapy should cover anaerobic bacteria.