The correlation for diastolic and systolic blood pressure was studied in two samples of quartets each consisting of two pairs. The first sample comprised pairs of sisters and their husbands, and the second sample was comprised of brothers and their wives. All siblings were between 30 and 55 years of age and had been married for at least 5 years. It was found that unrelated men married to sisters had a significant correlation in both diastolic (r = 0.28) and systolic (r = 0.41) pressure. For systolic blood pressure, the correlation between pairs of unrelated men married to sisters was significantly larger than the homologous correlation existing in pairs of brothers married to unrelated women. The correlations of systolic and diastolic pressure in sisters were significantly smaller than the same correlations measured in the wives of brothers. The correlations in height for men, used as an internal control to compare marital and genetic effects, were unaffected by marriage, as expected. The correlations in height for pairs of sisters, however, were no larger than those observed in pairs of unrelated women married to brothers. It was concluded that in adult married men and women of the Ferrara population, aged 30 to 55 years, the influence of genetic factors on blood pressure is less important than the influence of cultural factors.