A-mode ultrasonography is proving to be a useful procedure in an increasing number of anatomic sites and pathologic conditions. While the application of A-mode ultrasonography for determination of the position of the midline brain structures has already gained wide acceptance, there are other A-mode applications which afford considerable practical usefulness. Echo fetal cephalometry for measurement of the fetal biparietal diameter in utero is an accurate procedure, as is echo aortography for measurement of the lumen and wall thickness of the abdominal aorta. In a series of 275 proven cases,* A-mode ultrasonic differentiation of solid and cystic [See figures in the PDF file] masses has, in our hands, achieved an accuracy of 97 per cent. The usefulness of A-mode ultrasonography in localizing cysts and effusions preparatory to needle aspiration of fluid has been noted. The degree of diminution in size of such fluid collections after aspiration can also be recorded.