Most cerebrospinal fluid (CSF) shunts employed in the treatment of hydrocephalus include in their designs a pump mechanism that has been used in office practice as an indicator of whether the shunt is functioning properly; however, the pumping characteristics of shunts have been shown to correlate poorly with clinical status at a single point in time. A longitudinal study was performed to determine whether interval changes in the pumping characteristics of shunts correlate with changes in clinical status. 115 patients were examined on at least two occasions by a single pediatric neurosurgeon. At the time of the second examination, 10 patients were experiencing symptomatic shunt malfunction confirmed by neuroimaging investigations and operative findings; the remaining 105 patients were unchanged clinically at the time of the second examination. Considered as a test for CSF shunt malfunction, the interval shunt pumping test had a sensitivity of 0.50, a specificity of 0.64, and a likelihood ratio of 1.39. In the sample of patients studied, the negative predictive value was 0.93, and the positive predictive value was only 0.12. The interval shunt pumping test is not sensitive enough to rule out the possibility of shunt malfunction, nor can a positive interval shunt pumping test be considered an indication for an expensive diagnostic investigation in the asymptomatic patient.