Most cases of cerebral palsy in term infants are unexplained. We studied the association between chorioamnionitis and cerebral palsy in term and near-term infants. We performed a case-control study nested within the cohort of 236,378 singleton infants who weighed more than 2,000gm or who were older than 36 weeks born at Northern California Kaiser hospitals in 1991 to 1998. Potential cases identified electronically were children with inpatient or outpatient International Classification of Diseases Ninth Revision codes pertaining to cerebral palsy; we reviewed charts to confirm the diagnosis of moderate/severe spastic or dyskinetic cerebral palsy and to exclude infants with a known genetic abnormality or brain malformation. Controls were randomly selected from the study cohort. Clinical chorioamnionitis was defined as a physician diagnosis of chorioamnionitis/endometritis made before or after delivery on clinical grounds. Maternal delivery records were reviewed in a blinded fashion, and multivariate analysis was performed with logistic regression. Among the 62 cases, 53 had been diagnosed by a neurologist. Hemiparesis (30) was most common, followed by quadriparesis ( ) and paraparesis (10). Clinical chorioamnionitis was present in 6 of 62 cases versus 7 of 191 controls (odds ratio [OR], 2.8; p ฯญ 0.06), and histological chorioamnionitis was diagnosed in 4 of 62 cases versus1 of 191 controls (OR, 13; p ฯญ 0.01). After adjusting for maternal race, age, and preeclampsia, the presence of clinical or histological chorioamnionitis (relative risk [RR], 3.4; 95% confidence interval [CI], 1.1-10.4) and maternal black ethnicity (RR, 3.1; 95% CI, 1.0 -10.2) remained significantly associated with cerebral palsy. Chorioamnionitis is an independent risk factor for idiopathic cerebral palsy in term and near-term infants but is absent in most cases.