Moraxella osloensis, a gram-negative bacterium that is saprophytic on skin and mucosa, rarely causes infections. Moreover, infections in patients with cancer have not been reported. We describe 10 cases of M. osloensis blood or catheter infections that occurred during anticancer chemotherapy with or without preexisting neutropenia. The organism was identified definitively by sequencing analysis of the 16S ribosomal RNA gene. Fever (up to 39.7 degrees C) with substantial neutrophilia characterized these infections. The infections were monomicrobic for 3 patients and polymicrobic for 7 patients. Nine patients acquired the infection through central venous catheter colonization. The likely sources of the organism were sinusitis (3 cases), bronchitis (1 case), presumed subclinical mucositis from anticancer therapy (4 cases), and cutaneous graft-vs-host disease (2 cases). The infections resolved, without catheter removal, after antibiotic therapy with cell wall-active agents, to which all strains were shown to be susceptible. The M. osloensis strains exhibited significant morphologic variations on gram stain, and sheep blood agar was the preferred culture medium for 9 strains.