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To determine concordance for hypertension in adult male twins and to examine individual environmental factors associated with the manifestation of the disease in one member but not both members of a twin pair.The subjects were 1003 monozygotic and 858 dizygotic Caucasian, male, World War II veteran twins born in the USA between 1917 and 1927, who were aged 56–66 years when surveyed by the National Heart, Lung, and Blood Institute for health behaviors and cardiovascular disease status. Hypertensive status was determined by the subjects' diagnostic reports from physicians and the subjects' past or current use of antihypertensive medications. Self-reports were validated in a subsample of 675 individual twins who were participants in a series of cardiovascular examinations and for whom blood pressure measurements were available.The data analyses indicate that in this cohort of adult male twins, 62% of monozygotic and 48% of dizygotic cotwins of the hypertensive twins were hypertensives, compared with a prevalence of 36% in the whole cohort. Among the 281 monozygotic twins discordant for hypertension, hypertensive twins differed significantly from their non-hypertensive cotwins in the weight gain throughout adulthood and in alcohol consumption. They did not differ in weight at induction into the military, or according to smoking, physical activity or demographics. Those pairs that were concordant for hypertension gained significantly more weight throughout adulthood, consumed more alcohol, and were physically less active than concordant-negative pairs.The findings from this study suggest that although genetic factors influence the development of hypertension, non-genetic and potentially modifiable lifestyle behaviors, including adult weight gain, alcohol consumption and physical activity, are closely related to the clinical manifestation of the disease.