The effect of hydrocephalus on somatosensory evoked potentials was studied in nine infants. An increase in N1 latency was found in five infants studied longitudinally during a period of progressive ventricular dilatation. A marked decrease in N1 latency was noted in 7 infants, within one week following shunt insertion and in two infants who showed spontaneous arrest of ventricular growth. A correlation was found between cerebrospinal fluid pressure and the delay in N1 latency, but the number of infants studied is still small. SEPs appear to be a useful additional test when assessing infants with progressive ventricular dilatation. Once a baseline value for N1 has been obtained following shunt insertion, SEPs may subsequently be useful when assessing a child with possible shunt dysfunction.