We report a case of severe oral infection with a high fever due to Lactobacillus rhamnosus during induction chemotherapy for acute myeloid leukemia. The patient did not improve on treatment with meropenem, clindamycin, or vancomycin until neutrophil recovery. Since L. rhamnosus GG is used in dairy products, and the patient ingested dairy products daily before starting chemotherapy, we suspected an association between the ingestion of dairy products and the development of infection. Pulsed-field gel electrophoresis using two different restriction enzymes showed that the strain isolated from the patient was identical to the L. rhamnosus GG strain isolated from dairy products and ATCC #53103. This was confirmed by a PCR assay with species-specific L. rhamnosus GG primers. Since Lactobacillus infection, particularly L. rhamnosus infection, can be fatal in immunocompromised hosts, we should consider Lactobacillus as a causative organism when Gram-positive rods are detected during treatment with broad-spectrum antibiotics and vancomycin. The causal association between the ingestion of dairy products containing Lactobacillus and Lactobacillus infection in immunocompromised hosts warrants further study.