Postoperative endomyometritis develops in as many as 85% of women undergoing cesarean section, which is 10- to 30-fold higher than after vaginal delivery. The timing and mechanism by which the infecting organisms gain access to the uterine cavity are unclear. One possibility is that the infection may occur postpartum by ascending colonization of the wound. Alternatively, myometritis may be already present at the time the cesarean section is performed in asymptomatic patients. With tissue necrosis, the stage is then set for puerperal endomyometritis. To study this second alternative mechanism, myometrial and placental biopsy specimens were obtained in 91 asymptomatic patients at the time of cesarean section. Histologic evidence of chorioamnionitis was identified in 10% ( 9 91) of patients. Acute myometritis was present in 11% ( 10 91) of the myometrial biopsy specimens. Seven of the nine women (77%) with subclinical acute chorioamnionitis demonstrated extension of the inflammation into the myometrium. Thirty-two percent ( 8 25) of women in labor and 31% ( 5 16) of those with rupture of membranes for >6 hours had acute chorioamnionitis or myometritis, which is significantly higher (p < 0.01) than in women without these risk factors. These findings suggest that approximately one third of asymptomatic women with rupture of the membranes for more than 6 hours or who are in labor at the time of cesarean section demonstrate histologic evidence of subclinical chorioamnionitis. In most of these patients the myometrium is also involved. The uterine incision is then performed through infected myometrium, possibly setting the stage for puerperal endomyometritis. © 1988.