The study has a case-control design. Beginning in July 1990, through Emergency Medical Service incident reports, all cases of primary cardiac arrest, ages 25-74, without prior clinical heart diseases, were identified in Seattle and suburban King County, Washington during the period April 1990 to March 1994. Controls were identified from the same community using random digit dialing. Blood specimens from cases and controls were analyzed to determine red-cell membrane fatty acid composition. Spouses of cases and controls were interviewed to assess dietary intake of long-chain omega-3 PUFAs (a function of the amount of seafood consumed), usual fat consumption, changes in dietary intake, and presence or absence of other risk factors for primary cardiac arrest. Analyses were conducted to assess the relation between dietary intake and red-cell membrane omega-3-fatty acid levels and to assess whether there was a threshold or dose response effect for each fatty acid.