AbstractAn ultrasonic technique, using a modified A-scan instrument, for measuring the thickness of the post-mastectomy chest wall is described and results of multiple measurements on 46 patients who had undergone a simple or a radical mastectomy are presented and analysed. The average postmastectomy chest wall in the mid-clavicular line had a minimum thickness of 1·1 cm and a maximum thickness of 1·9 cm. There was little difference, within the main operative field, between patients with simple and radical mastectomies. Considerable variations in thickness were noted within the same patient and in the group as a whole.The technique is suitable for making chest wall measurements for planning radiotherapy with electrons.