To examine influence of insulin resistance and other clinical risk factors for the MetS (metabolic syndrome) on vascular structure and function in young adults. This cross-sectional study was conducted in a cohort of young adults (mean age 22 years) and their siblings participating in a longitudinal study of cardiovascular risk ( n =370). Insulin sensitivity was determined by euglycaemic insulin clamp. EDD (endothelium-dependent dilation) was determined by flow-mediated dilation using high-resolution ultrasound imaging of the brachial artery. EID (endothelium-independent dilation) was determined by NTG (nitroglycerine)-mediated dilation. The diameter and cIMT (intima–media thickness) of the carotid artery were also measured. There was no significant difference between males and females for age or body mass index. However, males had significantly higher glucose and triacylglycerol (triglyceride) levels, while the females had significantly higher HDL-C (high-density lipoprotein-cholesterol) and insulin sensitivity (13.00±0.33 compared with 10.71±0.31 mg·kg −1 of lean body mass·min −1 , P <0.0001). Although peak EDD was significantly lower (6.28±0.26 compared with 8.50±0.28%, P <0.0001) in males than females, this difference was largely explained by adjustment for brachial artery diameter ( P =0.15). Peak EID also was significantly lower in males than females (20.26±0.44 compared with 28.64±0.47%, P <0.0001), a difference that remained significantly lower after adjustment for brachial artery diameter. Males had a significantly greater cIMT compared with females (females 0.420±0.004 compared with males 0.444±0.004 mm, P =0.01), but when adjusted for carotid diameter, there was no significant difference ( P =0.163). Although there were gender differences in vascular function and structure in the young adult population examined in this study, many of the differences were eliminated simply by adjusting for artery diameter. However, the lower EID observed in males could not be explained by artery diameter. Future studies need to continue to examine influence of gender on EID and other measures of vascular function.