The patterns of blood pressures and their correlates in 499 Saudi nationals aged 10-72 years living at high altitude (3150 m above sea level) and 594 Saudi nationals aged 10-76 years living at low altitude (500 m above sea level) were prospectively studied. The average BPs were significantly higher in highlanders compared with lowlanders. In both high and lowland residents the BPs rose with age although males in both groups showed a drop in both SBP and DBP beyond the age of 50 years. Hypertension (> 160/95 mmHg) was not detected among lowlanders and only 1.4% of all highlanders were found to be hypertensive. These differences in BPs are attributed to the significantly higher body mass index values and packed cell volume levels in the highlanders.