Cine magnetic resonance (MR) is an established method for determining cardiac ventricular volumes in adults. The reported section slice thickness may be too thick for accurate measurement of ventricular volumes in infants and young children. In this article, acceptable slice thickness is established for cine MR in children. Balloons filled with a dilute gadolinium solution were immersed in a water bath. The balloons were scanned using a fast scan technique to simulate cine MR. Scanning was performed using different slice thicknesses. Volumes were calculated using previously reported techniques. These calculated volumes were compared to direct measurement of the balloon volumes using graduated syringes. The balloons varied in volume from 0.9 mL to 27.3 mL. Measurements using 3- and 5-mm slice thickness sections were found to be significantly more accurate than 10-mm slice thickness sections. Section thickness should be less than 1-cm when performing cine MR in infants and small children for measurement of ventricular volumes.