The relationship between smoking and the risk of hospitalization for acute myocardial infarction (MI) was evaluated among 121,964 nurses and aged 30-55 years who resided in 11 of the larger US states and who responded to a mail questionnaire. Among 249 women who experienced an MI, 159 (64%) were smokers at the time of hospitalization. Of 4977 controls matched to the cases on the basis of age, 1850 (37%) were smoking at the corresponding time. Smokers experienced a three-fold increase in risk of MI relative to individuals who never smoked, which was not explained by history of hypertension, diabetes, high cholesterol or familial MI. Women who had stopped smoking experienced a risk of MI no greater than women who had never smoked.