OBJECTIVES--To identify and rank the factors that currently limit the use of thrombolytic treatment in patients admitted to hospital with acute myocardial infarction. DESIGN--Weighted sampling study with retrospective data retrieval from clinical records. SETTING--All hospitals within the Trent region providing acute general medical services. PATIENTS--Random sample of 420 patients admitted during February-April 1993 who had acute myocardial infarction as the main discharge diagnosis. MAIN OUTCOME MEASURES--Treatment odds ratios (and 95% confidence intervals (CI)) for the use of thrombolysis in patient groups defined by relevant clinical characteristics. RESULTS--The patient population was older and less likely to have ST segment elevation on the initial electrocardiogram than patients entered into the randomised trials of thrombolysis. Thrombolytic treatment was given to 49% of patients (SE 2.4%). After controlling for negative associations with a history of stroke (treatment odds ratio 0.18 (95% CI 0.04 to 0.53)) and peptic ulcer (odds ratio 0.52 (95% CI 0.26 to 1.01)) use of thrombolysis decreased with increasing patient age. This was particularly noticeable for those aged > 74 years (odds ratio 0.17 (95% CI 0.05 to 0.51)) relative to those aged < 65 years. Thrombolysis was less likely to be used in patients with ST depression (odds ratio 0.22 (95% CI 0.11 to 0.41)) or bundle branch block (odds ratio 0.18 (95% CI 0.07 to 0.44)) than in those with ST elevation on the initial electrocardiogram. Delay from symptom onset to admission was more than 12 h in 15% of patients. CONCLUSIONS--The patient population admitted to hospital with acute myocardial infarction differs in several respects from the samples that have been included in the trials of thrombolysis. The main factors limiting wider use of thrombolysis are diagnostic uncertainty at admission and delayed presentation. Perceived clinical contraindications to treatment are of lesser importance. There is evident reluctance to use thrombolytic treatment in older patients, who were substantially under-represented in the clinical trials.