BACKGROUND Low molecular weight heparin can be administered by the subcutaneous route and has a stable and prolonged antithrombotic effect. These features have prompted clinical essays about its use as an alternative to unfractionated heparin in the treatment of unstable angina. OBJECTIVE To compare the clinical effects of low molecular weight heparin and unfractionated conventional heparin in patients with unstable angina or non Q infarction. METHODS Seventy patients (47 male) admitted to the hospital with the diagnosis of unstable angina or non Q acute myocardial infarction were randomly assigned to receive unfractionated intravenous heparin or subcutaneous low molecular weight heparin bid. All received aspirin p.o. and i.v. nitroglycerin. The incidence of recurrent angina, acute myocardial infarction or a need for emergency surgical revascularization during hospital stay were assessed in both groups. RESULTS Compared to patients with low molecular weight heparin, patients receiving unfractionated heparin had a higher incidence of recurrent resting angina (23 and 47.5% respectively, p < 0.04) and higher need for emergency surgical revascularization (3.3 and 17.5% respectively, p < 0.06). Patients treated with unfractionated conventional heparin had a 3 times higher risk of having an adverse cardiovascular event than patients receiving low molecular weight heparin (O.R. 0.33, confidence intervals 0.11-0.58). CONCLUSIONS Low molecular weight heparin is superior to unfractionated conventional heparin in the treatment of unstable angina and non Q acute myocardial infarction.