The basic requirement for declaring an association study positive is that the "hypertension-favoring" allele is more frequent in hypertensive cases than in normotensive controls. However, both positive and negative associations with hypertension have been found for the same polymorphism when studied in different populations. In the present study, we addressed the question of the possible cause(s) of this discrepancy among populations by using the alpha-adducin polymorphism as a paradigm. Four hundred ninety hypertensives and 176 normotensives enrolled in Sassari, Italy, and 468 hypertensives and 181 normotensives enrolled in Milano, Italy, were genotyped for the alpha-adducin Gly460Trp polymorphism. The blood pressure response to 2 months of hydrochlorothiazide therapy could be evaluated in 143 (85 in Sassari and 58 in Milano) hypertensives with and without the 460Trp alpha-adducin allele. The alpha-adducin 460Trp allele was not significantly more frequent in hypertensives in the Sassari population but was more frequent in hypertensives than in normotensives in Milano (P=0.019). Basal plasma renin activity was lower and blood pressure fall after diuretic therapy more pronounced (P<0.01) in hypertensives carrying at least one 460Trp allele than in Gly460Gly homozygotes, irrespective of their membership in the Sassari or Milano cohort. The effect of alpha-adducin genotype in predicting basal plasma renin activity and blood pressure decrease with diuretic treatment is similar in Sassari and Milano, despite the lack of association of the alpha-adducin genotype with hypertension in Sassari.