CONTEXT: Experimental data suggest that placental growth factor (PlGF), a member of the vascular endothelial growth factor family, acts as a primary inflammatory instigator of atherosclerotic plaque instability and thus may be useful as a risk-predicting biomarker in patients with acute coronary syndromes (ACS). OBJECTIVE: To determine whether blood levels of PlGF predict risk for death or nonfatal myocardial infarction in patients with acute chest pain. DESIGN, SETTING, AND PATIENTS: Measurement of PlGF levels as well as levels of markers of myocardial necrosis (troponin T [TnT]), platelet activation (soluble CD40 ligand [sCD40L]), and inflammation (high-sensitivity C-reactive protein [hsCRP]) in an inception cohort of 547 patients with angiographically validated ACS participating in the CAPTURE (c7E3 Fab Anti-Platelet Therapy in Unstable Refractory Angina) trial and in a heterogeneous cohort of 626 patients presenting with acute chest pain to an emergency department in Germany between December 1996 and March 1999. MAIN OUTCOME MEASURE: Risk for death or nonfatal myocardial infarction after 30 days. RESULTS: In patients with ACS, elevated PlGF levels (>27.0 ng/L; 40.8% of patients) indicated a markedly increased risk of events at 30 days (14.8% vs 4.9%; unadjusted hazard ratio [HR], 3.34; 95% confidence interval [CI], 1.79-6.24; P<.001). In a multivariable model, elevated levels of TnT (HR, 1.83; 95% CI, 1.05-3.86; P =.03), sCD40L (HR, 2.65; 95% CI, 1.41-4.99; P =.002), and PlGF (HR, 3.03; 95% CI, 1.54-5.95; P<.001) were independent predictors, while elevated hsCRP level was not (HR, 0.98; 95% CI, 0.53-1.98; P =.94). In patients with acute chest pain, elevated levels of PlGF predicted risk (21.2% vs 5.3%) (unadjusted: HR, 4.80; 95% CI, 2.81-8.21; P<.001; adjusted: HR, 3.00; 95% CI, 1.68-5.38; P