Importance: Higher baseline high-sensitivity C-reactive protein (hsCRP) levels after an acute coronary syndrome (ACS) are associated with adverse cardiovascular outcomes. The usefulness of serial hsCRP measurements for risk stratifying patients after ACS is not well characterized. Objective: To assess whether longitudinal increases in hsCRP measurements during the 16 weeks after ACS are independently associated with a greater risk of a major adverse cardiac event (MACE), all-cause death, and cardiovascular death. Design, Setting, and Participants: Secondary analysis of the double-blind, multicenter, randomized clinical Vascular Inflammation Suppression to Treat Acute Coronary Syndromes for 16 Weeks (VISTA-16) trial conducted between June 1, 2010, and March 7, 2012 (study termination on March 9, 2012), which included 5145 patients from 362 academic and community hospitals in Europe, Australia, New Zealand, India, and North America assigned to receive varespladib or placebo on a background of atorvastatin treatment beginning within 96 hours of presentation with an ACS. The present study evaluated data from patients with available baseline and longitudinal hsCRP levels measured at weeks 1, 2, 4, 8, and 16 after randomization to treatment or placebo. Statistical analysis was performed from June 15, 2018, through September 15, 2018. Main Outcomes and Measures: Outcomes were MACE (composite of cardiovascular death, myocardial infarction, nonfatal stroke, or unstable angina with documented ischemia requiring hospitalization), cardiovascular death, and all-cause death after adjustment for baseline clinical, treatment, and laboratory characteristics, including baseline hsCRP levels. Results: Among 4257 patients in this study, 3141 (73.8%) were men and the mean age was 60.3 years (interquartile range [IQR], 53.5-67.8 years). The median 16-week low-density lipoprotein cholesterol level was 64.9 mg/dL (IQR, 50.3-82.3 mg/dL), and the median hsCRP level was 2.4 mg/L (IQR, 1.1-5.2 mg/L). On multivariable analysis, higher baseline hsCRP level (hazard ratio [HR], 1.36 [95% CI, 1.13-1.63]; P =.001) and higher longitudinal hsCRP level (HR, 1.15 [95% CI, 1.09-1.21]; P <.001) were independently associated with MACE. Similar significant and independent associations were shown between baseline and longitudinal hsCRP levels and cardiovascular death (baseline: HR, 1.61 per SD [95% CI, 1.07-2.41], P =.02; longitudinal: HR, 1.26 per SD [95% CI, 1.19-1.34], P <.001) and between baseline and longitudinal hsCRP levels and all-cause death (baseline: HR, 1.58 per SD [95% CI, 1.07-2.35], P =.02; longitudinal: HR, 1.25 per SD [95% CI, 1.18-1.32], P <.001). Conclusions and Relevance: Initial and subsequent increases in hsCRP levels during 16 weeks after ACS were associated with a greater risk of the combined MACE end point, cardiovascular death, and all-cause death despite established background therapies. Serial measurements of hsCRP during clinical follow-up after ACS may help to identify patients at higher risk for mortality and morbidity.