Hypertensive complications of pregnancy are more common at high than low altitudes. Hypertension in pregnancy is associated with increased maternal and fetal morbidity and mortality; thus natural selection may be operating against women who develop the disorder and their infants. It has long been hypothesized that chronic hypoxia due to residence at high altitude predisposes women to develop hypertension during pregnancy. Prior studies indicate that maternal adaptation to pregnancy is altered by residence at high altitude such that some physiological characteristics of women pregnant at high altitude resemble those of women who develop hypertension during pregnancy at low altitude. This paper reviews data from studies, conducted over the past decade, which support a relationship between chronic hypoxia and hypertension during pregnancy. The results suggest that both the requirements for successful adaptation to high altitude, as well as the effect of lowered PO2 during pregnancy, may contribute to the development of hypertension during pregnancy. © 1995 Wiley-Liss, Inc.