Paraffin-embedded histological material was examined from 10 placentae from uncomplicated pregnancies at high altitude (3000 m). This was compared with material from 10 placentae delivered at low altitude (500 m). The sample groups were matched for maternal age, gestational age and parity. Within terminal and intermediate villi the volume-weighted mean cytotrophoblast cell volume did not significantly change at high altitude (754.1 microm3 at low altitude versus 796 microm3 at high altitude). The fractional volume of the villi occupied by cytotrophoblastic cells and their nuclei number per 10000 microm3 of villous tissue were significantly greater in placenta from high altitude (3.17 and 1.86 per cent, respectively) than those from low altitude (1.05 and 0.79 per cent, respectively) (P<0.0004 and P<0.0058, respectively). No significant differences in either fractional volume of the syncytiotrophoblast or its nuclei number per 10000 microm3 of villous tissue were observed between placentae from high (26.01 and 11.6 per cent, respectively) and low altitude (26.33 and 11.89 per cent, respectively). These results suggest an increase in the number of cytotrophoblastic cells at high altitudes without any changes in their volume. Exposure to hypobaric hypoxia is thought to be the principal aetiological factor.