A cohort of 166 twin pairs (67 monozygotic and 99 dizygotic) born at the Jackson Memorial Hospital/University of Miami Medical Center between July 1, 1976 and December 31, 1980, was followed from birth to one year of age in order to estimate the genetic variance of blood pressure during the first year of life. The sex-adjusted summary estimate of heritability for systolic blood pressure during all of infancy was 0.22 (p less than 0.001), but statistically significant genetic variance was not found for diastolic blood pressure. When using blood pressures from six to 12 months of age, adjusted for infant sex, heritability was estimated as 0.33 (p less than 0.001) for systolic blood pressure and 0.24 (p = 0.04) for diastolic blood pressure; adjustment for body weight reduced these estimates to 0.27 (p less than 0.001) and 0.17 (p = 0.13), respectively.