OBJECTIVE To determine whether amniotic fluid levels of pentraxin 3 (PTX3) are of value in the prenatal diagnosis of acute histological chorioamnionitis in preterm premature rupture of membranes (PPROM). METHODS Forty pregnant women with PPROM between 24 and 36 weeks of pregnancy without (n=21) and with (n=19) histological chorioamnionitis (PPROM group) and 42 women between 16 and 20 weeks of pregnancy (midtrimester group) were included in the study. We compared amniotic fluid PTX3 levels in the PPROM group with versus without histological chorioamnionitis, and between the PPROM and the midtrimester groups using nonparametric tests (Mann-Whitney test), given the non-normal distribution of the analyte. RESULTS Patients with histological chorioamnionitis had a significantly higher median amniotic fluid PTX3 concentration than patients without the histological signs of chorioamnionitis (3.69ng/mL [0.51-106.8] versus 0.8ng/mL [0.36-121.0]; P=0.015). Patients in the PPROM group reached a significantly higher median amniotic fluid concentration of PTX3 compared with those in the midtrimester group (1.0ng/mL [0.36-121.0] versus 0.67ng/mL [0.4-2.8]; P=0.007). CONCLUSIONS Histological chorioamnionitis is associated with a significant increase of amniotic fluid pentraxin 3 levels. Amniotic fluid pentraxin 3 appears to be a marker of intra-amniotic inflammation.