Accurate and efficient echocardiographic on-line determination of left ventricular volume would be advantageous in the care of children with congenital heart disease and children with hemodynamic instability. The prospective study was performed to evaluate the clinical usefulness of the on-line automatic border detection system (acoustic quantification: AQ) for determination of left ventricular volumes and ejection fraction in comparison to the conventional off-line method (manual tracing). 107 patients were enrolled in the study. The ages ranged from 0.1 to 18.8 years (mean 8.3 +/- 5.6). All patients were studied in the apical four-chamber plane for acoustic quantification (AQ) and manual tracing as well. Left ventricular volumes were determined using the mono-plane Simpson's rule. Left ventricular end-diastolic volumes obtained by AQ correlated well but were slightly underestimated compared to those determined by manual tracing (r = 0.99). Left ventricular endsystolic volumes by AQ correlated well but were also slightly underestimated compared to those obtained by manual tracing (r = 0.98). Mean ejection fraction was 61.1 +/- 6.8% by AQ compared with 61.5 +/- 5.9% by manual tracing. Linear regression analysis demonstrated good correlation: y = 0.77x + 14.1, r = 0.89; p < 0.001. Measurement of left ventricular volumes and ejection fraction by AQ using automatic border detection compares well with measurements done by manual tracing. However, AQ tends to underestimate to some degree. The time necessary for acquisition of data was similar in both methods. AQ seems to be a promising method for real-time estimation of left ventricular volume, even in children.