Chlamydia pneumoniae has been found within atherosclerotic plaques, and elevated titers of antibody to this organism have been linked to a higher risk of coronary events. Pilot studies have suggested that antibiotic treatment may reduce the risk of cardiovascular events. We enrolled 4162 patients who had been hospitalized for an acute coronary syndrome within the preceding 10 days and evaluated the efficacy of long-term treatment with gatifloxacin, a bactericidal antibiotic known to be effective against C. pneumoniae, in a double-blind, randomized, placebo-controlled trial. Subjects received 400 mg of gatifloxacin daily during an initial 2-week course of therapy that began 2 weeks after randomization, followed by a 10-day course every month for the duration of the trial (mean duration, 2 years), or placebo. The primary end point was a composite of death from all causes, myocardial infarction, documented unstable angina requiring rehospitalization, revascularization (performed at least 30 days after randomization), and stroke. A Kaplan-Meier analysis revealed that the rates of primary-end-point events at two years were 23.7 percent in the gatifloxacin group and 25.1 percent in the placebo group (hazard ratio, 0.95; 95 percent confidence interval, 0.84 to 1.08; P=0.41). No benefit was seen in any of the prespecified secondary end points or in any of the prespecified subgroups, including patients with elevated titers to C. pneumoniae or C-reactive protein. Despite long-term treatment with a bactericidal antibiotic effective against C. pneumoniae, no reduction in the rate of cardiovascular events was observed. Copyright 2005 Massachusetts Medical Society.