The association between low birth weight and high blood pressure is well established, but underlying mechanisms remain undefined. Vascular rarefaction, which may elevate peripheral vascular resistance, has been observed in capillaries of young men at risk for hypertension and men who had low birth weight. We looked for evidence that capillary rarefaction explains the association of low birth weight with high blood pressure in two cohorts. Participants in study 1 included 107 healthy boys aged 6 to 16 years recruited at random from a single school. Study 2 included 61 members of a cohort recruited at birth and studied at age 24 years. Measurements included indices of current size, blood pressure by automated sphygmomanometer, and dermal capillary density by video capillaroscopy of dorsal index finger skin after 10 minutes of venous occlusion. Lower birth weight predicted higher systolic blood pressure in both studies: in study 1, 3.57 mm Hg/kg birth weight (after adjustment for current height, 95% confidence interval 0.38 to 6.75, P<0.05); in study 2, 122+/-12 mm Hg in low birth weight (<2 kg) versus 115+/-9 in controls (P<0.05). Dermal capillary density was not associated in either group with birth weight or systolic blood pressure. We have found no evidence in these 2 cohorts that reduced capillary density explains the associations between lower birth weight and higher blood pressure.