The ingestion of a high fat meal induces postprandial lipaemia (PPL) and is associated with endothelial dysfunction. Both of these responses have been implicated in the atherosclerotic process, which has its origin in youth. Exercise has been shown to augment endothelial function and attenuate PPL, however the effect of exercise intensity on postprandial endothelial function in adolescents is unknown. This study examined the effect of a single bout of work-matched high-intensity interval exercise (HIIE) and moderate intensity exercise (MIE) performed 1 hour before a high fat meal on endothelial function, PPL and total antioxidant status (TAS) in adolescent boys. Ten boys (14.3 ± 0.3 y) completed three 1-day trials at least 1 week apart, and in a randomized order: 1) rest (CON); 2) 8 x 1 min cycling at 90% peak power with 75 s recovery (HIIE); 3) cycling at 90% of the gas exchange threshold (MIE), 1 hour before consuming a high fat milkshake (HFM), which provided 1.50 g kg-1 fat and 80 kJ kg-1. High-resolution ultrasound was used to assess flow-mediated dilation (FMD) of the brachial artery immediately before and 1 hour after exercise, and 3 hours after the HFM (4 hours post exercise). TAS was also assessed at these times. Plasma triacylglycerol (TAG) was measured before and after exercise, and every hour for 3 hours after the HFM. Mean differences in %FMD, TAG, and TAS between trials over time were analysed using a repeated measures ANOVA, with follow up comparisons interpreted using 95% confidence intervals (CI) and effect sizes (ES). An ES of 0.2 was deemed to be the smallest meaningful change. Baseline %FMD was comparable across all trials (P>0.05, ES<0.20). Compared to baseline, %FMD increased 1 hour after HIIE (P=0.02, 95% CI 0.6 to 4.2, ES=1.02), with no change in MIE (P=0.26, 95% CI -0.6 to 0.2, ES=0.11) and CON (P=0.33, 95% CI -0.2 to 0.5, ES=0.10). Compared to baseline, %FMD 3 h after the HFM was elevated in HIIE (P=0.03, 95% CI 0.2 to 3.2, ES=1.00), unchanged in MIE (P=0.60, 95% CI -0.5 to 0.9, ES=0.06), and reduced in CON (P=0.01, 95% CI -3.5 to -0.9, ES=1.16). Consequently, postprandial %FMD was greater in HIIE compared to MIE (P=0.003, 95% CI 0.9 to 2.6, ES=1.03) and CON (P<0.001, 95% CI 2.3 to 4.8, ES=1.88), and in MIE compared to CON (P=0.004, 95% CI 0.8 to 2.7, ES=0.88). No differences in total or incremental area under the curve for plasma TAG (P>0.05, ES<0.20) or TAS (P>0.05, ES<0.20) versus time were apparent between trials. These data indicate that an acute bout of exercise performed 1 hour before a high fat meal may have a protective effect on the vasculature, and that this effect may be intensity dependent. Specifically, postprandial endothelial function can be preserved after MIE, and improved after HIIE. These effects were independent of meaningful changes in PPL and TAS after exercise.