Abnormal amniotic fluid volume may be both the result and the cause of complications at various stages of pregnancy. This review focuses on evidence from randomised trials of the effectiveness of amnioinfusion. Attempts to prevent the development of pulmonary hypoplasia by means of transabdominal amnioinfusion to correct severe oligohydramnios in the middle trimester of pregnancy have not been evaluated by randomised trials. Long-term transcervical amnioinfusion for prelabour rupture of the membranes has shown promising results in prospective studies, but has also not been subjected to randomised evaluation. Transcervical amnioinfusion during labour is a relatively simple procedure. Normal saline is infused through an intrauterine catheter, preferably one with a catheter-tip pressure transducer. Once an adequate volume of amniotic fluid has been achieved, it is maintained by means of a slow continuous infusion or repeated bolus infusions. Amnioinfusion has been used to prevent or treat fetal heart rate (FHR) decelerations thought to be due to oligohydramnios. In several rather small randomised trials, amnioinfusion has been associated with a reduction in FHR decelerations, caesarean sections, low Apgar scores, low umbilical arterial pH values and postpartum endometritis. Amnioinfusion has also been used to dilute thick meconium-staining of the amniotic fluid. In trials for this indication, amnioinfusion has been associated with similar benefits, and in addition a reduction in meconium presence below the vocal cords, meconium aspiration syndrome and the need for neonatal ventilation. Larger trials are needed to determine whether amnioinfusion is associated with an effect on perinatal mortality, and whether complications such as umbilical cord prolapse are increased.