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The anatomical location of the third portion of the duodenum is between the superior mesenteric artery (SMA) and the abdominal aorta (AA). When the aorto-mesenteric angle (AMA) is small and the aorto-mesenteric distance (AMD) is short, the duodenum becomes tightly compressed between these two blood vessels. Severe compression can obstruct the duodenum, resulting in vomiting and abdominal pain. This clinical condition is termed superior mesenteric artery syndrome (SMA syndrome). The duodenum is frequently affected in Henoch–Schönlein purpura (HSP). The aim of the present study was to verify that duodenal anatomy mimicking SMA syndrome is a risk factor for duodenal involvement and abdominal pain in HSP.A prospective case–control study involving 12 HSP patients compared with 48 age- and sex-matched children (controls) was conducted. Bowel abnormalities were defined as thickened wall >3 mm and paralytic ileus detected on ultrasound. AMA, AMD and obesity index (OI) were compared between the two groups on Mann–Whitney testing. The correlation between duodenal bowel abnormalities and SMA syndrome was examined using Fisher's exact test.AMA, AMD and OI were significantly smaller in the HSP patients (P < 0.001, 0.003 and 0.026, respectively). All HSP patients, but only 10 controls, met the ultrasound diagnostic criteria for SMA syndrome (100% vs 20.8%, P < 0.001).Duodenal compression by SMA against the AA, which mimics SMA syndrome, may lead to duodenal bowel abnormalities with abdominal pain in thin children with HSP.