Considerable advances have been made in clinical pharmacology of late, permitting more precise evaluation of new drugs. This must be taken into account in assessing the thrombolytic therapy of acute myocardial infarction. Clinical trials necessitate control groups, which must in volve a sufficiently large number of patients so that differences are not due to chance. To be valid, the groups must be strictly comparable. It is essential to define precisely criteria for admission and contraindications, if possible stratifying the patients, since the final evaluation must include all patients according to the treatment originally allocated. For recent myocardial infarction, it is important to compare thrombolytic therapy with the best treatment currently available, the patients being randomly allocated to these alternatives. However, it is not clear whether the best current treatment is anticoagulation, polarizing solutions (combining glucose, insulin and potassium), prophylactic antiarrhythmic drugs (procainamide, lignocaine) or β-adrenergic inhibitors (propranolol), and therefore which to use in comparison. The trend to coronary care units already has shown a reduction in mortality, perhaps necessitating evaluation of thrombolytic therapy only in such units. If thrombolysis finds a place in the management of myocardial infarction, treatment must start within the first hours following onset of symptoms. This increases the likelihood that patients diagnosed erroneously also will be treated with thrombolytic agents. Reassurance is needed that such treatment is harmless in these circumstances. Lastly, the statistical interpretation of the value of thrombolytic therapy must be regarded as a ‘two-tailed’ situation.