Cross-sectional echocardiography is a new noninvasive technique for imaging the heart. We developed a method for using mechanical cross-sectional echocardiograms (sector scans) to determine left ventricular volumes and ejection fraction. Using left ventricular cineangiography as a standard, sector scan ejection fraction correlated better (r = 0.93) than M-mode echocardiography by any of three established methods, and the sector scan regression line did not differ from the line of identity (p > 0.33). Interobserver variability for sector scan ejection fraction was 2.3 i 1.2% (mean ± SD). Variation between two studies performed within 24 hours and analyzed by the same observer was 1.4 ± 1.5%. However, the sector scans consistently underestimated left ventricular end-diastolic volume. We conclude that sector scan echocardiography is more reliable than conventional M-mode techniques for estimating left ventricular ejection fraction, but estimation of left ventricular end-diastolic volume is unreliable with the methods currently available.