The M235T polymorphism of the angiotensinogen gene (AGT) has been associated with essential and pregnancy-induced hypertension. Generation of haplotypes can help to resolve whether the T235 allele itself predisposes to the development of hypertension or acts as a marker of an unknown causal molecular variant. We identified 10 diallelic polymorphisms at the AGT locus and genotyped both a series of 477 probands of hypertensive families and 364 controls, all French Caucasians, as well as a series of 92 hypertensives and 122 controls from Japan. Despite a large ethnic difference in gene frequency, a significant association of T235 with hypertension was observed both in Cancasians (.46 vs. .38, P = .004) and in Japanese (.91 vs. .76, P = .002). In both groups, the G-->A substitution located at position -6 upstream of the initial transcription site occurred at the same frequency and in complete linkage disequilibrium with the T235 allele. No other polymorphism was found to be consistently associated with hypertension. Five informative haplotypes subdividing the T235 allele were generated. Whereas two of them were associated with hypertension in Caucasians, none of these two haplotypes (H3 and H4) reached statistical significance in Japanese. The analysis of the AGT-GT repeat revealed marked linkage disequilibriums between each of the diallelic polymorphisms and some (GT)n alleles, with similar patterns in the two populations. The strong disequilibrium between M235 and (GT)16 explained the increased frequency of that particular allele in French controls compared with hypertensives (.42 vs. .36, P < .01). The haplotype combining the M235T and G-6A polymorphisms appears as the ancestral allele of the human AGT gene and as the one associated with hypertension.