The authors hypothesized that individuals born in the early 20th century who experienced the hottest and driest summers during infancy would be more likely to have suffered severe infant diarrhea and dehydration, and consequently have had higher blood pressure in adulthood, than those who experienced cooler and wetter summers. In this context, these climate data act as an instrumental variable for the association of early-life dehydration with later blood pressure. For 3,964 randomly selected British women born between 1919 and 1940 and whose blood pressure was measured at age 60-79 years, a one standard deviation (1.3 degrees C) higher mean summer temperature in the first year of life was associated with a 1.12-mmHg (95% confidence interval: 0.33, 1.91) higher adult systolic blood pressure, and a one standard deviation higher mean summer rainfall (33.9 mm) was associated with lower systolic blood pressure (-1.65 mmHg, 95% confidence interval: -2.44, -0.85). Equivalent results for diastolic blood pressure were 0.11 (95% confidence interval: -0.65, 0.86) and -0.32 (95% confidence interval: -0.71, 0.05). The climate variables were not associated with potential confounding factors such as socioeconomic position or lifestyle risk factors. These findings provide some evidence in favor of the hypothesis that dehydration in infancy is associated with higher adult blood pressure.