In a cross-sectional total population survey of Faizia East Primary Health District of Buraidah, which is divided into an "urban" and "rural" sections, casual blood pressure was recorded in 5671 subjects. 2222 (910 males) were adults (> 19 years). 3299 (1561 males) were between 3-18 years, making the paediatric/adolescent cohort of the population. The rest were below 3 years and were not included for calculations in this report. For adults, HPN was defined as > or = 140/90 mm Hg and P/A it was > or = 95th percentile for the age cohorts 3-5, 6-9, 10-12, 13-15 and 16-18 years as recommended by The Task Force for Blood Pressure Control in Children (1987). Each of the three subsets of HPN were derived from these basic definitions without modification. In the adults population, overall prevalence (S/DHPN + ISHPN + IDHPN) was 23.58% (524/2222) and gender prevalence was marginal in favour of males (25.71 vs 22.1% for males and females respectively; OR: 1.22, chi square = 3.89; p = 0.05; CI: 1.00 < OR < 1.49). By contrast, overall HPN prevalence in P/A was 10.64% (351/3299) and in all the age cohorts, girls were very significantly more hypertensive than boys. Overall figures were 13.06 vs 7.94% respectively for girls and boys; OR: 0.57; chi square = 22.65; p < 0000019; CI: 0.45 < OR < 0.73. Only in severe HPN (> or = 99th percentile) in 16-18 year age cohort did male preponderance become obvious. When the influence of the living environment was examined, in adult population (urban: 40,001; rural: 1670 subjects), for all age cohorts the rural environment which is largely inhabited by the unsophisticated Bedouins living close to their livestock, significantly predisposed to HPN as compared to the urban setting. Overall figures are 19.95 vs 32.60%, urban vs rural respectively: OR: 0.52; chi square = 40.4; p < 0.000000; CI: 0.42 < OR < 0.64. On the other hand, in the P/A population (2301 urban, 998 rural) for the age cohorts 3-5 and 6-9 years the urban environment significantly predisposed to HPN. At 10-12 years the prevalence was virtually even (12.65 vs 12.71%; OR: 0.99; p: NS). Thereafter, i.e. 13-15, 16-18 year cohorts, the rural environment became more dominant, just like the adult pattern. In other words whatever the factors in the unsophisticated Bedouin "rural" setting which predisposes to HPN does not become operative until after childhood--after 12 years. This contrasting gender and environmental influence in our study population seems unique, being reported for the first time. It may be pointing us in new direction in the elucidation of the pathogenesis of HPN and should deserve further studies.