OBJECTIVE--To describe blood pressure in twins during infancy. DESIGN--Prospective study of cohort of twins. SETTING--Teaching hospital in Florida. SUBJECTS--166 viable twin pairs born between July 1976 and December 1989. MAIN OUTCOME MEASURES--Blood pressure and body weight at birth, at 14 days, and at 1, 3, 6, 9, and 12 months. RESULTS--Both systolic and diastolic pressure correlated with body weight throughout infancy (at birth r = 0.41, P < 0.001 and r = 0.42, P < 0.001 respectively; at 1 year r = 0.23, P < 0.001 and r = 0.26, P < 0.001 respectively). In infants weighing < 1500 g at birth mean blood pressure rose from about 45/25 mm Hg to 101/55 mm Hg from birth to the age of 1 year, while in infants weighing > 3000 g at birth it rose from 63/39 mm Hg to 100/61 mm Hg; corresponding mean body weights at 1 year were 7.86 kg and 9.88 kg. Differences in birth weight within pairs of monozygotic twins were negatively correlated with such differences in systolic blood pressure at 1 year (r = -0.37, P < 0.01). CONCLUSIONS--Blood pressure and body weights in twins showed strongly positive but generally declining correlations throughout infancy. Twins of lower birth weight showed a more rapid rate of rise in blood pressure during infancy. At 1 year the catch up in blood pressure exceeded that in body weight. Greater differences in birth weights between monozygotic twins were associated with smaller differences in systolic blood pressure at 1 year, suggesting that intrauterine environmental factors related to birth weight are important in determining blood pressure in infancy.