In the early to mid 1980s, the WHO MONICA Project conducted cardiovascular risk factor surveys in 41 study populations in 22 countries. Study populations aged 35-64 years comprised 32,422 men and 32,554 women. Blood pressures (BP) and body mass index (BMI) were measured according to a standard protocol. Participants were asked about antihypertensive medication. In men, the average age-standardized BPs ranged among the populations from 124 to 148 mm Hg for systolic (SBP) and from 75 to 93 mm Hg for diastolic (DBP). The corresponding values in women were 118-145 mm Hg for SBP and 74-90 mm Hg for DBP. In all populations, women had lower SBP than men in the age group 35-44. However, SBP in women rose more steeply with age so that in 34 of 41 populations women had higher SBP than men in the age group 55-64. The proportion of participants with untreated major elevation of BP ranged from 4.5% to 33.7% in men and from 1.9% to 22.3% in women. The proportions of participants receiving antihypertensive medication were 4.3-17.7% for men and 6.0-22.0% for women. These proportions were not correlated with the prevalence of untreated hypertensives. Age-adjusted BMI was associated with SBP and accounted for 14% of the SBP variance in men and 32% in women. We found a large difference in SBP among the MONICA study populations and conclude that the results represent a valid estimate of the public health problem posed by elevated BP. We also have shown that almost universally the problem of elevated BP is more prevalent in women than in men, especially in the older age groups.