Objective. – To assess whether the dietary intake of long-chain n-3 polyunsaturated fatty acids from seafood, assessed both directly and indirectly through a biomarker, is associated with a reduced risk of primary cardiac arrest. Design. – Population-based case-control study. Setting. – Seattle and suburban King County, Washington. Participants. – A total of 334 case patients with primary cardiac arrest, aged 25 to 74 years, attended by paramedics during 1988 to 1994 and 493 population-based control cases and controls, matched for age and sex, randomly identified from the community. All cases and controls were free of prior clinical heart disease, major comorbidity, and use of fish oil supplements. Measures of Exposure. – Spouses of case patients and control subjects were interviewed to quantity dietary n-3 polyunsaturated fatty acid intake from seafood during the prior month and other clinical characteristics. Blood specimens from 82 cases (collected in the field) and 108 controls were analyzed to determine red blood cell membrane fatty acid composition, a biomarker of dietary n-3 polyunsaturated fatty acid intake. Results. – Compared with no dietary intake of eicosapentaenoic acid ([C.sub.20]:5n-3) and docosahexaenoic acid ([C.sub.22]:6n-3), an intake of 5.5 g of n-3 fatty acids per month (the mean of the third quartile and the equivalent of one fatty fish meal per week) was associated with a 50% reduction in the risk of primary cardiac arrest (odds ratio [OR], 0.5; 95% confidence interval [Cl], 0.4 to 0.8), after adjustment for potential confounding factors. Compared with a red blood cell membrane n-3 polyunsaturated fatty acid level of 3.3% of total fatty acids (the mean of the lowest quartile), a red blood cell n-3 polyunsaturated fatty acid level of 5.0% of total fatty acids (the mean of the third quartile) was associated with a 70% reduction in the risk of primary cardiac arrest (OR, 0.3; 95% Cl, 0.2 to 0.6). Conclusion. – Dietary intake of n-3 polyunsaturated fatty acids from seafood is associated with a reduced risk of primary cardiac arrest. (JAMA. 1995;274:1363-1367); Eating more fish may lower the risk of cardiac arrest. Fish contain long-chain n-3 polyunsaturated fatty acids, which can be incorporated into blood cell membranes and change their physiological properties. Researchers identified 334 people who had experienced a cardiac arrest between 1988 and 1994 in King County, Washington. They chose 493 healthy volunteers from the community as the control group. In both groups, people with heart disease were excluded. Spouses were asked to estimate fish intake and membrane fatty acid composition was measured in samples of the participants' blood. Eating the equivalent of two fish meals a month reduced the risk of cardiac arrest by 30% and the equivalent of one fish meal a week reduced the risk by 50% compared to those who ate no fish. Increased amounts of n-3 fatty acids in red blood cell membranes was also associated with a lower risk of cardiac arrest.