A new two-dimensional ultrasound imaging system capable of producing high resolution tomographic images of the heart in real time has been developed. This system relies on phased array principles to rapidly steer the ultrasound beam through the structures under investigation. A hand-held linear array of 16 ultrasound transducers with overall dimensions of 14 mm at the site of contanct may be readily manipulated to image various cardiac structures. The resulting images are displayed in a circular sector format, 60 degrees in azimuth and typically 15 cm in range. At his maximum range, image frames consisting of 256 lines are generated at the rate of 20 frames/second. High azimuthal resolution throughout the field of view is assured by a focused transmit beam and by sweeping the focus of the receiver is synchrony with the range of returning echoes. Azimuthal resolution varies from 2 to 5 mm throughout the field of view while range resolution is 1.5 mm. This imaging system has proven particularly useful for the delineation of left ventricular spatial geometry by the identification of endocardium, myocardium, papillary muscles and interventricular septum. High quality images of anterior and posterior mitral leaflets, aortic root and aortic leaflets as well as left atrium and other cardiac structures have been obtained.