The contribution of body mass index (BMI) at different life stages to adult blood pressure (BP) is not well known. We investigated whether changes in BMI across childhood and adulthood, and birthweight, influence BP in mid-adulthood. In 9297 participants in the 1958 British birth cohort, we analysed BMI measures at several ages (7, 11, 16, 33 and 45 years) and BP at 45 years. Positive BMI/BP associations strengthened with age; at 45 years a standard deviation (SD) increase in concurrent BMI was associated with an increase in systolic blood pressure (SBP) of 3.9 mmHg [95% confidence interval (CI) 3.6-4.2] and an elevated risk of hypertension (odds ratio 1.68; 95% CI 1.60-1.76). The effect of concurrent BMI on BP strengthened with decreasing childhood BMI; among the thinnest 10% at 11 years, SBP increased by 5.5 mmHg per SD increase in concurrent BMI; among the heaviest 10% the increase was 3.3 mmHg. Excessive BMI gain, especially recently, was associated with raised BP; allowing for BMI at 33 years, a SD increase in BMI between 33 and 45 years was associated with an SBP increase of 4.1 mmHg (95% CI 3.6-4.6). Birthweight was inversely associated with BP; SBP reduced by 1.3 mmHg (95% CI 0.9-1.7) per SD increase in birthweight, independently of BMI. High BMI and excessive BMI gain at any life stage, particularly recently, is associated with increased adult BP. Relatively thin children were vulnerable to BMI gain and increased BP in adulthood. Elevated BP associated with low birthweight was not caused by 'catch-up' growth.