Objective: Alterations in microvascular function have been hypothesized as a possible mechanism explaining the negative association of weight at birth with blood pressure and insulin resistance in adult life. However, these variables are closely associated, so that it has been difficult to establish whether microvascular dysfunction is a cause or a consequence of increased blood pressure or insulin resistance. Design: Cohort study. Setting: VU University Medical Center, Amsterdam, The Netherlands. Subjects: Twenty-one prepubertal healthy children showing a wide range in birth weight. Main outcome measures: Birth weight data were obtained from hospital records. Blood pressure was measured with an ambulatory 24-h blood pressure monitor, and insulin sensitivity was assessed with the hyperinsulinaemic euglycaemic clamp technique. Microvascular function (i.e. capillary recruitment during post-occlusive reactive hyperaemia and endothelium (in)dependent vasodilatation of the skin) was evaluated by videomicroscopy and iontophoresis of acetylcholine and sodium nitroprusside. Results: Birth weight was positively and significantly associated with capillary recruitment [slope, 22%/kg birth weight; 95% confidence interval (Cl), 0.1-43; P<0.05]. Birth weight was not associated with insulin sensitivity and systolic blood pressure (slope, -0.11 mg/kg per min per pmol/l; 95% Cl, -2.4 to 2.2; P= 0.9; and slope, 1.4 mmHg; 95% Cl, -5.0 to 7.7/kg birth weight; P= 0.7, respectively). The association between low birth weight and impaired capillary recruitment was not affected by adjustment for blood pressure and insulin sensitivity. Birth weight was not associated with endothelium-(in)dependent vasodilatation. Conclusion: These results suggest that the association between birth weight and capillary recruitment is independent of blood pressure and insulin sensitivity. These findings are consistent with the hypothesis that an impaired capillary recruitment plays a mechanistic role in the association of birth weight with blood pressure and insulin resistance in adult life.