Objective The relationship between low birth weight and elevated blood pressure in adult life is well established but presently unexplained. Both microvascular dysfunction and insulin resistance have been proposed as a possible explanation. We have examined the relation between birth weight and blood pressure in 30 healthy subjects exhibiting a wide range of insulin sensitivity, and assessed whether microvascular function and/or insulin resistance may underlie this relationship. Methods Birth weight data were obtained from birth announcements. Blood pressure was measured with an ambulatory blood pressure monitor and insulin sensitivity was assessed by the hyperinsulinaemic, euglycaemic clamp technique. Microvascular function, i.e. capillary recruitment and endothelium-dependent and -independent vasodilatation in the skin, was evaluated by videomicroscopy and iontophoresis of acetylcholine and sodium nitroprusside. Results Birth weight was significantly associated with blood pressure (r= -0.50; P < 0.05), capillary recruitment (r= +0.52; P< 0.05), acetylcholine-mediated vasodilatation (r= +0.40; P < 0.05), insulin sensitivity (r= +0.62; P< 0.01) and waist-to-hip ratio (r= -0.42; P< 0.05). Regression analysis showed a significant association of birth weight with 24 h systolic blood pressure (regression coefficient: -7.6 mmHg/kg; 95% confidence interval: -13.0 to -1.0). Adjustment for capillary recruitment and waist-to-hip ratio decreased the regression coefficient by 39 and 41%, respectively. The results were similar after adjustment for age, sex or body mass index. Conclusion These results suggest that capillary recruitment and body fat distribution may partly explain the relationship between birth weight and blood pressure, (C) 2000 Lippincott Williams and Wilkins.