We evaluated the relation between smoking and myocardial infarction using data from a hospital-based case-control study conducted in Northern Italy on 262 young and middle-age women with acute myocardial infarction (median age: 49 years) and 519 controls (median age: 47 years), admitted for a series of acute conditions unrelated to any of the established or potential risk factors for ischaemic heart disease. With reference to lifelong non-smokers, the multivariate relative risk was not significantly higher for ex-smokers, but rose progressively with the number of cigarettes smoked. The risk estimates were 2.3, 5.9 and 11.0 for less than 15, 15-24, greater than or equal to 25 cigarettes per day. This trend in risk was statistically significant. Smoking-related risks were similar below and above 50 years of age; they were consistently and substantially higher in various strata of other major determinants of myocardial infarction (hyperlipidaemia, hypertension) or correlated lifestyle habits (alcohol and coffee). In terms of population attributable risk, 48% of all myocardial infarction in this data could be attributed to smoking. Although myocardial infarction is less frequent in Italian women compared to Northern European or American women, our data indicate that cigarette smoking is undoubtedly its most prominent cause. This confirms, once again, the urgent need to intervene and eliminate this risk factor.