Eighteen hydrocephalic children who presented with subtle deterioration when their shunts malfunctioned were studied during shunt revision by means of the pressure-volume index (PVI) technique. Bolus manipulation of cerebrospinal fluid (CSF) was used to determine the PVI and the resistance to the absorption of CSF (Ro). Ventricular size was moderately to severely enlarged in all the children. Steady-state intracranial pressure (ICP) at the time of shunt revision was 17.5 +/- 7.3 mm Hg (range 8 to 35 mm Hg). Pressure waves could not be induced by bolus injections in the 8- to 35-mm Hg range of ICP tested. The mean +/- standard deviation (SD) of the predicted normal PVI for this group was 18.5 +/- 2.7 ml. The mean +/- standard error of the mean of the measured PVI was 35.5 +/- 2.1 ml, which represented a 187% +/- 33% (+/- SD) increase in volume-buffering capacity (p less than 0.001). The ICP did not fall after bolus injections in three children, so that the Ro could not be measured. In the remaining 15 patients, Ro increased linearly as a function of ICP (r = 0.74, p less than 0.001). At ICP's below 20 mm Hg, Ro ranged from 2.0 to 5.0 mm Hg/ml/min, but increased to as high as 21 mm Hg/ml/min when ICP was above 20 mm Hg. This study documents that subtle deterioration in shunted hydrocephalic children is accompanied by abnormally compliant pressure-volume curves. These children develop ventricular enlargement and neurological deterioration without acute episodic pressure waves. The biomechanical profile of this group differs from other children with CSF shunts.