In this study the prevalence of bladder dysfunction in 18 children with gross bilateral reflux was investigated via cystometric recordings. In all except 1 infant maximal voiding detrusor pressure was 100 cm. or greater water (range 86 to 244). Pronounced instability during filling (overt instability) with pressure waves above baseline (mean 65 cm. water, range 42 to 194) was found in 9 infants. Another 5 children had evidence to suggest an uninhibited bladder, not manifested as unstable contractions during filling but as covert instability, meaning that the first unstable contraction was transformed into a premature and forceful voiding contraction. The high detrusor pressures found in 18 children with gross bilateral reflux during the voiding phase but also during the filling phase in half of the children suggest that a hypercontractile detrusor may be a contributory factor for the development of reflux even in this age group.