ObjectiveTo investigate the association between birth size and hypertension within a genetically homogenous population of high birth weight.DesignCohort-study with retrospectively collected data on size at birth.Subjects and settingThe study included 4601 men and women born 1914–1935 in Reykjavik, Iceland, who participated in the Reykjavik Study of the Icelandic Heart Association.Main outcome measuresBirth size measurements, adult blood pressure (BP) and body mass index (BMI), and family history of hypertension.ResultsBirth weight was inversely related to hypertension in adulthood in women (Pfor trend<0.001). The relationship was of borderline significance in men (Pfor trend = 0.051). A low ponderal index was significantly associated with high BP in women (Pfor trend = 0.025) but not men (P>0.05). For women with an adult BMI>26 kg/m2, the odds ratio for hypertension for those born weighing<3.45 kg was 2.1 [95% confidence interval, 1.3–3.3, compared with women born weighing>3.75 kg. The association was only significant in women without a family history of hypertension.ConclusionsAn inverse association between size at birth and adult hypertension was seen in a population of greater birth size than has previously been investigated. The relation was strongest among women born small who were overweight in adulthood, and for those without a family history of hypertension. The results support the hypothesis that the association between birth weight and hypertension is not of genetic origin only. The large birth size of Icelanders might be protective and partly explain the lower mean systolic blood pressure in Iceland than in related nations.