OBJECTIVE To determine physicians' attitudes toward prescribing acetylsalicylic acid (ASA), physicians' own use of ASA and the prevalence of ASA use in the community following the trials of ASA for primary prevention of coronary heart disease. METHODS Random sample surveys of physicians and the general public by mail and telephone respectively and a mail survey of a selected panel of expert cardiologists and neurologists. METHODS London, Ont., and surrounding Middlesex County. METHODS A total of 210 physicians (77% of eligible subjects), including family practitioners and most types of specialists, with an active medical licence and 666 English-speaking people (75% of eligible subjects) aged 18 years or more living in a household with active, listed telephone service. METHODS Long-term ASA use (at least 80 mg on alternate days for 4 or more consecutive weeks) for the treatment of atherosclerosis. RESULTS Sampled physicians and experts agreed that long-term ASA therapy was indicated in patients with unstable angina, a transient ischemic episode or recent myocardial infarction but not for primary prevention in healthy middle-aged men and women at low risk for ischemic vascular disease. Both groups were uncertain about the role of ASA in primary prevention in asymptomatic people with risk factors for atherosclerosis. Nine (16%) of the 55 male physicians aged 50 years or more took ASA routinely for primary prevention. In the community survey almost all those who used ASA routinely were 50 years or older. The proportions of men and women in this age group who used ASA routinely for any reason were 19% (95% confidence limits [CLs] 11 and 28) and 14% (95% CLs 8 and 19) respectively; the proportions of men and women who used ASA routinely and apparently for primary prevention were 8% and 1% respectively. A total of 43% (95% CLs 30 and 57) of those with apparent ischemic vascular disease took ASA routinely. Medically unsupervised long-term ASA use for primary or secondary prevention of ischemic vascular disease was uncommon (reported by 2% of those who used the drug routinely). CONCLUSIONS Physicians generally agree on a role for long-term ASA therapy in the secondary prevention of ischemic vascular disease. However, the prevalence of long-term ASA use in people with overt atherosclerosis in the community may be less than optimal. The role of the drug in the primary prevention of ischemic vascular disease is less accepted. Long-term ASA use in the community for primary prevention is uncommon but detectable.