OBJECTIVE To determine independent risk factors for endometritis after low transverse cesarean delivery. METHODS We performed a retrospective case-control study during the period from July 1999 through June 2001 in a large tertiary care academic hospital. Endometritis was defined as fever beginning more than 24 hours or continuing for at least 24 hours after delivery plus fundal tenderness in the absence of other causes for fever. Independent risk factors for endometritis were determined by means of multivariable logistic regression. A fractional polynomial method was used to examine risk of endometritis associated with the continuous variable, duration of rupture of membranes. RESULTS Endometritis was identified in 124 (7.7%) of 1,605 women within 30 days after low transverse cesarean delivery. Independent risk factors for endometritis included age (odds ratio [OR] for each additional year, 0.93 [corrected] [95% confidence interval {CI}, 0.90-0.97]) and anemia or perioperative blood transfusion (OR, 2.18 [CI, 1.30-3.68]). Risk of endometritis was marginally associated with a proxy for low socioeconomic status, lack of private health insurance (OR, 1.72 [CI, 0.99-3.00]); with amniotomy (OR, 1.69 [CI, 0.97-2.95]); and with longer duration of rupture of membranes. CONCLUSIONS Risk of endometritis was independently associated with younger age and anemia and was marginally associated with lack of private health insurance and amniotomy. The odds of endometritis increased approximately 1.7-fold within 1 hour after rupture of membranes, but increased duration of rupture was only marginally associated with increased risk. Knowledge of these risk factors can guide selective use of prophylactic antibiotics during labor and heighten awareness of the risk in subgroups at highest risk of infection.