At the time cesarean section, amniotic fluid was collected transabdominally from 60 patients, and quantitative cultures were performed on the amniotic fluid. A culture was defined as positive if greater than or equal to 10(2) colony-forming units per milliliter of a high-virulence organism were isolated. Any other result was defined as negative. In 24 patients with no labor or rupture of the membranes, no positive cultures were found, but there was a 25% incidence of endometritis. Among 36 patients with labor or rupture of the membranes, or both, 13 (36%) had a positive culture. Twelve of the 13 (92%) developed endometritis, whereas nine of the 23 (39%) patients with a negative culture had endometritis (p less than 0.002). The usual clinical risk factors for endometritis were not different between the positive and negative culture groups. However, the patients with positive cultures had a significantly shorter time interval from cesarean section to endometritis than did the patients with negative cultures (p less than 0.02). There was an excellent correlation between a positive amniotic fluid culture and endometritis after cesarean section.