Newer brands of cigarettes with reduced yields of nicotine and carbon monoxide have been promoted as being less hazardous than others. We evaluated the effect of smoking "low yield" cigarettes on the risk of nonfatal myocardial infarction in women under 65 years of age. The data were obtained in a case-control study of 910 women with a first myocardial infarction and 2375 hospital controls. The estimated relative risk for current smokers as compared with those who had never smoked increased with the number of cigarettes smoked. The estimated overall relative risk was 3.7 (95 percent confidence interval, 3.0 to 4.7). The estimated risks did not vary according to the nicotine or carbon monoxide yield of the cigarette. The estimated relative risk (4.7) in women who smoked brands with the lowest levels of nicotine (less than 0.40 mg per cigarette) was similar to that (4.2) in smokers of the higher-yield brands (greater than 1.30 mg). For exsmokers, the estimated relative risk was 1.4 (95 percent confidence interval, 1.0 to 1.8). These data suggest that women who smoke low-yield cigarettes do not have a lower risk of a first nonfatal myocardial infarction than women who smoke higher-yield brands. For smokers who wish to reduce their risk, switching to low-yield brands is a poor alternative to quitting.