Risk factors for listeriosis are foods and underlying diseases. A case-control-study of listeriosis patients' dietary habits showed that unpasteurized milk was a risk food for sporadic listeriosis. A blue-mould cheese was significantly more often eaten by patients ill with an epidemic phage type. Leukaemia, AIDS and renal transplantation were found to be associated with a more than 1000 times higher risk of acquiring listeriosis, compared with healthy persons. As a part of prophylaxis, persons with a high risk of listeriosis should be informed individually about food hygiene and risk foods.