The effect of altitude on amniotic fluid volume has not been studied previously. We sought to determine whether there is such an effect by measuring the amniotic fluid index (AFI) in pregnant women at low and high altitudes. We compared populations of pregnant women at two institutions, one located at low elevation (100 ft) and the other at relatively high altitude (6,000 ft). Uncomplicated singleton pregnancies between 20 and 42 weeks' gestation with adequate dating criteria were studied. Patients were excluded if any condition known to alter amniotic fluid volume was present. Data were stratified by gestational age at two-week intervals for analysis. The pregnant women at high altitude (n = 364) had a significantly increased (P < .001) AFI throughout gestation as compared to the low-altitude group (n = 514). The mean difference in the AFI between the two populations for the two-week intervals was 1.5 cm (95% confidence interval, 1.0-2.0). As compared to the low-altitude group, the high-altitude population had a greater proportion of women with polyhydramnios (5.8% versus 1.0%, P < .001) and fewer patients with oligohydramnios (2.5% versus 5.6%, P < .001) after categorization of AFI values utilizing previously established nomograms. This finding suggests that there is an association between high altitude and an increase in the AFI. The mechanism and clinical significance of this effect are unknown.