Quantitative isotope ventriculography (QIV) has been suggested as a method for diagnosing hydrocephalus and predicting the results of CSF shunt insertion. However, the physiological significance of this and other tracer clearance studies is questionable. Using a simplified mathematical model for tracer clearance following ventricular injection, a series of isotope retention curves were generated by an IBM 360-40 computer. The author shows that the tracer clearance from the CSF is just as dependent on the volume of the CSF compartment as it is on the rate of CSF absorption. This means, for example, that doubling the CSF volume without diminishing absorption (cerebral atrophy) yields the same results as halving the absorption rate in patient with normal CSF volume (early hydrocephalus). In order for tracer clearance studies to measure CSF absorption, to determine the presence of hydrocephalus, and to predict the results of shunting, an accurate measure of CSF volume must be obtained.