Previous studies have demonstrated a high correlation between hydrocephalus and the resistive index (RI), as determined by transcranial Doppler ultrasonography. Measurements of RI, calculated by dividing the difference between the peak systolic velocity and the end-diastolic velocity by the peak systolic velocity, were attempted in 55 pediatric patients during evaluations for ventriculoperitoneal shunt malfunctions; values were obtained in 52. Indications of shunt malfunction included both clinical and radiographic evidence of increased intracranial pressure. Eleven patients, determined to have functional shunts both by clinical criteria and subsequent outcome, had RIs of 47 +/- 5 (average +/- 1 standard deviation). Shunt malfunctions were confirmed in 41 patients. Prior to shunt revisions, these 41 patients had RIs of 71 +/- 10%; following revision, the RIs fell to 53 +/- 12%. Nine patients had had pre-malfunction RIs of 48 +/- 11% obtained during routine follow-ups; when they subsequently had shunt malfunctions, their RIs had significantly increased. Four of the 41 patients with shunt malfunctions had essentially normal RIs (52 +/- 7%), but had fluid tracking along the shunt; in these, RIs were essentially unaffected by shunt revision. For comparison, 119 pediatric patients with clinically functional ventriculoperitoneal shunts had RIs of 50 +/- 9%. The data, statistically significant with a P value of less than 0.001, showed a correlation between elevated RIs and shunt malfunction; thus, transcranial Doppler ultrasonography is a practical, non-invasive technique useful in the diagnosis of ventriculoperitoneal shunt malfunction.