Inconsistent findings linking placental histologic chorioamnionitis (HCA) and preterm delivery may result from variations in HCA definition, population studied, and exclusion criteria. This analysis from the 1998-2004 Pregnancy Outcomes and Community Health Study (five Michigan communities) includes the first 1,053 subcohort women (239 preterm, 814 term) with completed placental assessments. Multiple HCA definitions were constructed by 1) varying polymorphonuclear leukocytes/high-powered field thresholds and placenta components included and 2) using polymorphonuclear leukocyte characteristics to assign low/high maternal, fetal inflammation stage and grade. In African Americans, HCA was associated with preterm delivery before 35 weeks. The effect size was modest for polymorphonuclear leukocytes/high-powered field thresholds of greater than 10 and greater than 30 (odds ratios (ORs) = 0.8 and 2.0); larger for greater than 100 (OR = 3.2, 95% confidence interval (CI): 1.4, 7.1); strengthened after excluding medically indicated preterm deliveries (OR = 4.9, 95% CI: 2.0, 11.8); and strongest for high maternal/high fetal HCA (OR = 5.6, 95% CI: 1.4, 22.1). These latter HCA criteria also produced the largest effect size in Whites/others (OR = 2.7, 95% CI: 0.3, 26.9). Among preterm deliveries before 35 weeks excluding those medically indicated, 12% of Whites/others and 55% of African Americans had high maternal HCA. The authors conclude that HCA definition, exclusion criteria, and race/ethnicity influence the HCA-preterm delivery association and that HCA contributes to preterm delivery-related ethnic disparity.