Background Obesity is an independent risk factor for cardiovascular morbidity and mortality. Large artery compliance is thought to be associated with cardiovascular risk. The effect of weight loss on large artery compliance is not yet clarified. Objective To investigate the effect of weight loss, with or without exercise, on vessel wall properties in healthy obese men. Design This was a pair-matched randomized intervention study. All subjects were on an energy-restricted diet. One subject from each pair was also on an exercise programme. Measurements were performed before and at the end of the study period. The study lasted for 3 months. Methods The vessel wall properties of the brachial and common carotid artery were assessed using a vessel wall movement detector system in combination with applanation tonometry. Results The mean body mass index was 32.3 +/- 0.4 kg/m(2) and decreased (P < 0.001) to 27.6 +/- 0.4 kg/mm(2) during the study. The mean blood pressure decreased (P < 0.001) by 6%. At operating pressures, cartotid artery distensibility was 27.5 +/- 1.7 x 10(-3)/kPa at the start of the study and 31.1 +/- 1.8 x 10(-3)/kPa (P < 0.04) at the end of the study. Brachial and carotid artery compliances were 0.11 +/- 0.01 and 1.35 +/- 0.08 mm(2)/kPa at the start of the study and tended to increase to 0.12 +/- 0.001 (P = 0.06) and 1.48 +/- 0.08 mm(2)/kPa (P = 0.057), respectively, at the end of the study. Isobaric compliance did not change. The diet-and-exercise group did not differ statistically from the only-diet group in the effects on weight loss, blood pressure and arterial compliance. Conclusion This study shows that weight loss increased carotid artery distensibility at operating pressures, but not under isobaric conditions. This increase is probably due to the decrease in blood pressure. The addition of exercise did not result in an additional effect within 3 months. J Hypertens 1999, 17:1831-1835 (C) Lippincott Williams & Wilkins.