Cardiomyopathy syndrome (CMS) was first diagnosed in the mid-1980s in farmed Atlantic salmon in Norway, and later also in Scotland and the Faeroe Islands. In Norway the number of diagnosed cases increased from 25 to 103 in the period from 1998 to 2001 according to the National Veterinary Institute (NVI) records. Very little is known about the causes of the disease and there is no documentation of its impact on the farmed salmon industry. This field survey was performed to collect information on occurrence, risk factors and the economic importance of CMS in sea farmed Atlantic salmon Salmo salar in Norway. Data were collected in January 2001 from a total of 174 groups of farmed salmon which had been previously transferred to sea during 1999 and 2000. Approximately 11.5% of all groups of salmon in our study experienced cases of CMS. Affected fish were presumably in generally good condition prior to time of death. In fish which had completed the seawater production cycle, CMS occurred more than 1 yr (median 395 d, >2 kg body weight) after seawater transfer. In fish transferred into the sea during autumn 2000 which had not completed the seawater cycle during the study period, CMS was diagnosed at a weight as low as 700 g. Although sudden death is characteristic, CMS may be regarded as a chronic disease, with moderately elevated mortality rates at site level. Affected groups showed significantly increased mortality, causing a direct annual financial loss for the industry of Euros 4.5 to 8.8 million on fish farms. Preventive prescheduled slaughtering of salmon, which is performed on many affected farms, may be too costly when done too early.