1. The familial resemblance in causal blood pressure was studied in material derived from an ophthalmic population survey. The population consisted of 1078 individuals forming 373 families and 1410 pairs divided into groups with different relationships. In order to adjust for age, the difference between the logarithm of the systolic blood pressure and that of the value expected from a linear regression on age was used instead of the observed pressure. 2. There was no significant correlation between spouses. Children did not resemble their fathers at all. Sons and daughters were significantly correlated with their mothers. The resemblance between siblings was even greater and highly significant. The familial resemblance was not dependent on synchronized pressure readings or on a common household and was not related to the time available for cumulation of effects. 3. Together these findings formed a pattern strongly suggesting long-lasting maternal effects on the systolic blood pressure.