IPN (infectious pancreatic necrosis) is a disease well-known on Danish pond farms for a considerable time. During recent years the IPN virus has been frequently isolated from fish on Norwegian farms. However, it has caused clinical signs of disease only in a few cases. In Sweden the disease appeared in 1969, was eliminated but re-appeared on several farms in 1985. In Finland the IPN virus was isolated from rainbow trout in a sea cage farm in 1984; the fish did not show any clinical symptoms. Whether the recent increaase in IPN virus isolation is due to the spread of the disease or due to increased screening for the virus remains to be determined.