We investigated the relationship between weight at birth and the prevalence of hypertension (defined as treatment and/or systolic blood pressure > 160 mmHg) at ages 50, 60 and 70 years in a cohort of Swedish men followed longitudinally, in which we had previously found a strong inverse association of birthweight with blood pressure at age 50. In men of above median adult height (> 176 cm), a 1000 g decrease in birthweight was associated with an odds ratio for hypertension of 2.53 (95% CI 1.10, 5.85) at age 50, 1.63 (95% CI 0.97, 2.72) at 60 and 1.72 (95% CI 1.10, 2.69) at 70. As the overall prevalence of hypertension increased steeply with age, the absolute difference in the risk of hypertension between those of low and high birthweight increased with age. In men of below median height, birthweight was not significantly associated with blood pressure at any age. Small size at birth is associated with hypertension at 50, 60 and 70 years in men of a high growth potential. The strength of the relationship between birthweight and hypertension does not increase from 50 to 70 years, although the risk of hypertension attributable to low birthweight does increase with age.