An outbreak of Listeria monocytogenes Serotype 3a occurred among recent patients of a tertiary care facility (hospital), in Finland. This was an unusual strain of Listeria. An investigation found the outbreak strain of Listeria monocytogenes in packaged butter. Delivery of the butter to the hospital began just before the first cases were identified. The patients were more likely to have had a malignancy or to have been an organ recipient and were, as a result, more susceptible to Listeria infection.