Objective--To study the effect of birth weight and body weight on blood pressure in late adolescence. Design--Analysis of data on weight, height, and blood pressure at age 17 of subjects from the Jerusalem perinatal study, according to their birth weight. Data for men and women were analysed separately. Setting--Jerusalem, Israel. Subjects--32,580 subjects (19,734 men and 12,846 women) born in the three major hospitals in Jerusalem during 1964-71 and subsequently drafted in to the army. MAIN OUTCOME MEASURES--Correlations between birth weight and blood pressure at age 17 and weight and height at age 17 and blood pressure. Results--Diastolic and systolic blood pressures were associated with birth weight in both young men and young women, but the correlation coefficients were low. A high body weight at age 17 (greater than 66 kg for women, greater than 75 kg for men) rather than a low birth weight (less than 2500 g) was linked with higher systolic and diastolic blood pressures in both men and women (p less than 0.01). Conclusions--Intrauterine environment, as reflected by birth weight, has little effect on blood pressure in young men and women. Modification of factors which lead to excess weight during adolescence may have a major role in preventing hypertension in adults.