Abstract. Ultrasonography was used to measure the width of the internal os in 96 pregnant women, of whom 43 were considered to have cervical incompetence and were scheduled for cerclage. The remainder of the women were divided into two control groups: (a) 28 women with a normal obstetric history, and (b) 25 women with a history of either previous abortion or traumatic delivery. The mean width of the internal os in the three groups was 25.9 mm±3.7, 15.8 mm±2.7, and 18.2 mm±6.5 respectively; statistically, significant difference were seen between the first group and each of the control groups. All the women in the first control group gave birth successfully at term; of those in the second control group, 5 presented with premature rupture of the membranes. The mean cervical os width in these 5 women was 29 mm, as compared with a mean of 18.2 mm for this group as a whole. We suggest that ultrasonic measurement of the internal os during pregnancy can be a useful and objective new method for the diagnosis of cervical incompetence.