To evaluate the effect of regular aerobic exercise on the endothelial function and cardiorespiratory fitness in pregnant women. This was a randomized, double-blinded, controlled clinical trial of 64 healthy primigravid women between 16 and 20 weeks' gestation. The experimental group took part in aerobic exercise at an intensity of 50-65% of their maximum heart rate for 60 min, three times a week for 16 weeks. The control group undertook their usual physical activity. The outcomes were the endothelial function evaluated by flow-mediated dilatation and cardiorespiratory fitness evaluated by the maximum indirect oxygen consumption (VO2max) in a 6-min walk test. Initially, no differences were found between the groups in any of the variables. At the end of the intervention, the participants that performed exercise showed a higher cardiorespiratory fitness, measured by the walked distance in the 6-min walk test (P = 0.014) and by the VO2max (P = 0.014). Also, the exercise group showed a lower heart rate at rest and a higher flow-mediated dilatation than the control group (P = 0.02). Regular aerobic exercise improves endothelium-dependent vasodilation in pregnancy. This intervention may be an early and effective alternative to strengthen the prevention of disorders associated to endothelial dysfunction. © 2011 The Authors. Journal of Obstetrics and Gynaecology Research © 2011 Japan Society of Obstetrics and Gynecology.