The effect of a full bladder containing 300 ml or more of urine was studied during normal established labor in 20 patients and 10 controls. Uterine activity was measured in Montevideo units, and the progress of labor was assessed by partogram before and after catheterization. Although uterine activity increased after catheterization, there was no significant change in the slope of the partogram. It is concluded that a full bladder does not affect the course of normal established labor.