To evaluate the association between subclinical and clinical chorioamnionitis and risk of preterm birth (PTB).Demographic and clinical characteristics were abstracted from medical records and placental examinations performed (N = 1371 pregnancies including spontaneous and medically-indicated PTBs). Pregnancies were classified as having clinical chorioamnionitis (with or without histologic chorioamnionitis), subclinical chorioamnionitis (histologic, but not clinical, chorioamnionitis), or no chorioamnionitis; pregnancies with histologic chorioamnionitis were further evaluated for fetal vasculitis. Relative risks for PTB, early and late PTB, and PTB ± premature rupture of membranes (PROM) were adjusted for maternal characteristics.Clinical (4.3%) and subclinical (24.5%) chorioamnionitis were not associated with PTB overall. In pregnancies without clinical or subclinical chorioamnionitis, the risk of PTB with PROM and early PTB was 2.2% and 8.6%, respectively. In comparison, clinical chorioamnionitis was associated with an increased risk of PTB with PROM (aRR: 3.42 (95%CI: 1.07, 10.98), whereas subclinical chorioamnionitis was associated with increased risk of PTB with PROM (aRR: 3.92 (95% CI: 2.15, 7.12)) and early PTB (aRR: 1.77 (95% CI: 1.18, 2.64)). Histologic chorioamnionitis with fetal vasculitis was associated with increased risk of PTB with PROM (aRR: 7.44 (95% CI: 3.68, 15.05)) and early PTB (aRR: 2.94 (95% CI: 1.78, 4.87)), whereas histologic chorioamnionitis without fetal vasculitis was associated with increased risk of PTB with PROM only (aRR: 2.64, 95% CI: 1.27, 5.50).Subclinical chorioamnionitis and histologic chorioamnionitis with fetal vasculitis were associated with early PTB and PTB with PROM but not with PTB overall, likely due to inclusion of indicated PTBs.