Eighteen cases of adult listeriosis (nine with meningitis, one brain abscess and eight bacteraemia) were diagnosed in the Chaim Sheba Medical Center in the years 1964-1982. The infection seemed to be opportunistic in all. Eleven patients had malignant disease, two had cirrhosis of the liver, one had ulcerative colitis, one had bronchial asthma with chronic obstructive pulmonary disease, one had pemphigus, one had diabetes mellitus and one had a renal transplantation. Twelve patients (66%) received radiation therapy and/or cytotoxic and steroid medication. Diabetes mellitus as an additional underlying disease was strikingly frequent and was found in eight out of 18 patients (44%), in one as the only underlying disease. In the meningitis group cerebrospinal fluid (CSF) cultures were positive in five patients, and negative in four who had, however, positive blood cultures. The cells in the CSF were predominantly lymphocytes in five and polymorphs in four. It may be concluded that diabetes mellitus is an important underlying disease in listeriosis. The results also reinforce the fact that lymphocytosis in the CSF does not exclude bacterial meningitis.