Intravenous heparin has been used in the control of myocardial ischaemia in patients with unstable angina. We set out to assess the efficacy of subcutaneous heparin in reducing myocardial ischaemia in patients with unstable angina. 343 of 399 patients with unstable angina were monitored for 24 h and 108 were refractory to conventional antianginal treatment and were entered into a randomised multicentre trial. 37 patients were assigned to heparin infusion (partial thromboplastin time 1.5-2 times baseline), 35 to subcutaneous heparin (adjusted dose with partial thromboplastin time 1.5-2 times baseline), and 36 to aspirin (325 mg daily). All had additional conventional antianginal therapy. After the run-in patients were monitored for 3 days. The primary endpoint was reduced myocardial ischaemia assessed by the number of anginal attacks, silent ischaemic episodes, and duration of ischaemia per day. At 1 week and 1 month we accounted for anginal attacks and other clinical events (myocardial infarction, revascularisation procedures, and death). Aspirin did not significantly affect the incidence of myocardial ischaemia. On the first 3 days, infused and subcutaneous heparin significantly decreased the frequency of angina (on average by 91% and 86%, respectively), episodes of silent ischaemia (by 56% and 46%), and the overall duration of ischaemia (66% and 61%) versus run-in day and aspirin (p < 0.001 for all variables). The favourable effects of heparin therapy remained evident during follow-up. Only minor bleeding complications occurred. Subcutaneous heparin is effective in the control of myocardial ischaemia in patients with unstable angina.