A functioning shunt must have a patent proximal catheter within the cerebrospinal fluid space. Occlusion with choroid plexus or ependymal tissue might be expected if these tissues are in contact with the proximal inlets. This study was undertaken to define the intraventricular distances available for a standard-placement shunt cather and to compare the available distances with actual ventricular catheter inlets. In 52 normal subjects (age range from 1 month to 20 years; median 7.7 years) magnetic resonance imaging was used to measure the dimensions of the anterior horn in planes typically used for cather placement. For anterior placements, the intraventricular length (ventricle entry to the foramen of Monro) was measured for a standardized catheter route ('As') and for a route yielding a maximum lenght ('Amax'). For posterior placements, the length (ventricle tip to foramen of Monro) was obtained for a standardized catheter placement to the ipsilateral ('Pi') and the contralateral ventricle ('Pc') as well as measuring a maximum length in a curved trajectory ('Pmax'). The average length (and range) in centimeters for the various trajectories was: As = 1.5 (1.1-1.9), Amax = 1.7 (1.2-2.2), Pi = 1.6 (1.2-2.1), Pc = 2.0 (1.4-2.9), and Pmax = 2.8/3.1 (2.1-3.6). Minor variations from a standard shunt insertion site did not affect the length of catheter within the ventricle. Current ventricular catheters have proximal inlets extending 1.6-2.4 cm from the catheter tip. Variations in standard ventricular catheter placement should have no measurable effect on how much ventricle is available for the proximal catheter. It may not be possible to place a standard ventricular catheter and keep the inlets within the ventricle and placed consistently away from choroid plexus and ependyma, regardless of approach. This may contribute to the similar proximal occlusion rates reported for the differing placements. Subsequent studies of shunt placement should correlate proximal occlusion rates with ventricular size. To prevent ventricular size from directly affecting proximal conclusion, consideration should be given to altering the design of ventricular catheters by placing inlets over a shorter distance (1.0 cm) from the tip.