Listeria monocytogenes infection, formerly considered a rather rare disease, had been reported increasingly. The infection occurs mostly in patients with underlying diseases, Such as tumor, diabetes mellitus, kidney transplantation and alcoholism, and also in patients with steroides and cytotoxic chemotherapy. In 1978 and 1979, 3 cases of infections of L. monocytogenes were bacteriologically diagnosed at Severance hospital, and the following observations were made. 1) The first patient was a 58-year-old male with malignant lymphoma, the 2nd a 66-year-old female with cervical cancer and the last a 46-year-old female with diabetes mellitus. 2) L. monocytogenes was isolated from blood specimens in all of the patients. From the 3rd patient a CSF specimen was also cultured and yielded the organism. 3) All of the isolates showed the typical cultural and biochemical characteristics of L. monocytogenes and were found to belong to serotype 1b. 4) All of the isolates were susceptible to ampicillin and tetracycline. Erythromycin and penicillin susceptibilities were variable depending on the isolates. Increasing incidence of listeriosis is expected in the future due to the ever increasing number of compromised hosts. It is thought necessary that both clinicians and laboratorians should be familiar with the bacterium and its infection to be able to make accurate diagnosis and to provide proper treatment.