AIMS: The safety and efficacy of abciximab in addition to low-molecular-weight-heparin as the primary medical treatment of acute coronary syndromes has not previously been investigated. METHODS AND RESULTS: The GUSTO IV-ACS trial included 7800 patients with chest pain and either ST-segment depression or a positive troponin test. They were randomized to abciximab for 24 h, 48 h or placebo. In the dalteparin substudy, 974 patients received 5 days of s.c. dalteparin, instead of a 48 h infusion of unfractionated heparin (UFH). Major and minor bleedings were more frequent for abciximab (24 and 48 h combined) than placebo both in the dalteparin (abciximab 5.0% vs placebo 1.8% P<0.05) and in the UFH cohort (3.8% vs 1.8% P<0.001). However, stroke rates were low, < or = 0.6%. At 30 days there were no significant differences in the rate of death or MI, either in the dalteparin (abciximab 9.6% vs placebo 11.3%: O.R. 0.85; 95% C.I. 0.58-1.25) or in the UFH cohort (8.5% vs 7.6%: O.R.; 1.12: 0.95-1.34). CONCLUSION: Treatment with abciximab, aspirin and s.c. dalteparin is associated with a low risk of major side effects and is as safe as the combination of abciximab and UFH. Without early coronary intervention there is no indication for abciximab treatment.