To determine if light to moderate consumption of alcohol influences the risk of primary cardiac arrest, the authors investigated 152 cases, 25-75 years of age, without prior heart disease or co-morbidity, in whom a primary cardiac arrest occurred during a 14-month period in King County, Washington state, December 1979-January 1981. Spouses of cases and those of 152 demographically similar residents of King County were interviewed to quantify average alcohol consumption over the prior year for beer, wine, and spirits combined. After adjustment for hypertension, smoking, and physical activity, light to moderate alcohol consumption was associated with a reduced risk of primary cardiac arrest. Compared to persons who were nondrinkers (less than 1 drink/month), the estimated relative risk for light drinkers (greater than or equal to 1 drink/month but less than 1 drink/day) was 0.7 (95% confidence interval, 0.5-1.0), and for moderate drinkers (1-3 drinks/day), 0.5 (95% confidence interval, 0.3-1.0). These data are consistent with the hypothesis that light to moderate alcohol consumption reduces the risk of primary cardiac arrest.