BACKGROUND Oligohydramnios (reduced amniotic fluid) may be responsible for malpresentation problems, umbilical cord compression, concentration of meconium in the liquor, and difficult or failed external cephalic version. Simple maternal hydration has been suggested as a way of increasing amniotic fluid volume in order to reduce some of these problems. OBJECTIVE The objective of this review was to assess the effects of maternal hydration on amniotic fluid volume and measures of pregnancy outcome. METHODS The Cochrane Pregnancy and Childbirth Group trials register and the Cochrane Controlled Trials Register were searched. Date of last search: 1 February 1999. METHODS Randomised trials comparing maternal hydration with no hydration in pregnant women with reduced or normal amniotic fluid volume. METHODS Eligibility and trial quality were assessed by both reviewers. RESULTS Two studies of 77 women were included. The women were asked to drink two litres of water before having a repeat ultrasound examination. Maternal hydration in women with and without oligohydramnios was associated with an increase in amniotic volume (weighted mean difference for women with oligohydramnios 2.01, 95% confidence interval 1.43 to 2.56; and weighted mean difference for women with normal amniotic fluid volume 4.5, 95% confidence interval 2.92 to 6.08). Intravenous hypotonic hydration in women with oligohydramnios was associated with an increase in amniotic fluid volume (weighted mean difference 2.3, 95% confidence interval 1.36 to 3.24). Isotonic intravenous hydration had no measurable effect. No clinically important outcomes were assessed in any of the trials. CONCLUSIONS Simple maternal hydration appears to increase amniotic fluid volume and may be beneficial in the management of oligohydramnios and prevention of oligohydramnios during labour or prior to external cephalic version. Controlled trials are needed to assess the clinical benefits and possible risks of maternal hydration for specific clinical purposes.