Transvaginal ultrasonic monitoring of the lower uterine segment was performed in 15 pregnancies in which the mother had previously had three or more caesarean sections. The typical findings were thinning of the myometrium, asymmetric isthmal structures and ventral ballooning of the isthmus. An image of the other of the two isthmal fenestrations was obtained by ultrasound. The antepartum ultrasonic findings and those found at operation correlated well. Maternal antepartum symptoms, on the other hand, poorly reflected the physical state on the scarred uterine isthmus.