In high risk patients with non ST elevation acute coronary syndromes(ACS), en oxaparin is generally preferred to unfractionated heparin(UFH). However, less is known about the relative merits of these two forms of heparin in patients recei ving concomitant glycoprotein IIb/IIIa inhibitors. The A phase of the A to Z t rial was an open label non inferiority trial in which 3987 patients with non S T elevation ACS were randomised to receive either enoxaparin or UFH in combinati on with aspirin and tirofiban. Inclusion required either ST depression or cardia c biomarker elevation. While the selection of an early management strategy (inva sive or conservative) was at the discretion of the local investigator, investiga tors were asked to designate their plans for an invasive or conservative strateg y on the case record form. An early conservative strategy was specified for 1778 patients(45%); this subgroup forms the population for the present analyses. Am ong patients with a planned conservative strategy, baseline characteristics were similar between those randomised to UFH (n=872) and those randomised to enoxapa rin (n=906). The primary endpoint of death, new MI, or documented refractory isc haemia within 7 days of randomisation occurred in 10.6%of patients randomised t o UFH and 7.7%of patients randomised to enoxaparin (HR 0.72; 95%CI 0.53-0.99; p=0.04). The combined rate of TIMI major, minor, or loss no site bleeding was 1.3%in patients treated with UFH and 1.8%in those treated with enoxaparin (p=n s). When a conservative approach to catheterisation and PCI was planned for ACS patients receiving tirofiban and aspirin, enoxaparin was associated with superio r efficacy and similar bleeding vs UFH. However, less is known about the relative merits of these two forms of heparin in patients recei ving concomitant glycoprotein IIb / IIIa inhibitors. The A phase of the A to Z ri ri was an open label non inferiority trial in which 3987 patients with non ST elevation ACS were randomized to receive either enoxaparin or UFH in combinati on with aspirin and tirofiban. Inclusion required either ST depression or cardia c biomarker elevation. While the selection of an early management strategy (inva sive or conservative) was at the discretion of the local investigator, investiga tors were asked to designate their plans for an invasive and conservative strateg y on the case record form. An early conservative strategy was specified for 1778 patients (45%); this subgroup forms the population for the present analyzes. Am ong patients with a planned conservati The primary endpoint of death, new MI, or documented refractory isc haemia within 7 days of randomization occurred in 10.6% of patients randomized to UFH and 7.7% of patients randomized to enoxaparin (HR 0.72; 95% CI 0.53-0.99; p = 0.04). The combined rate of TIMI major, minor, or loss no site bleeding was 1.3% in patients treated with UFH and 1.8% in those treated with enoxaparin (p = ns). When a conservative approach to catheterization and PCI was planned for ACS patients receiving tirofiban and aspirin, enoxaparin was associated with superior efficacy and similar bleeding vs UFH.