An 18-year-old cyclist was referred to the laboratory for unexplained pain in his left lower limb during exercise. He had started competition at 14 years of age and had cycled ≈10 000 km/y in the preceding 5 years. He had complained for 18 months of a subjective sensation of a painful and swollen left thigh at maximal exercise. Different physiotherapies and treatments proved unsuccessful. Clinical investigation and ankle-to-brachial indices at rest were normal (1.10 on the left side and 1.15 on the right side). An incremental (50 W/3 min) bicycle exercise test was stopped at 330 W because of pain. Ankle-to-brachial pressure measurements showed a fall of left-side values <0.5. Ultrasound imaging showed typical lesions of endofibrosis, consisting of a mildly echoic thickening of the left myointimal border (normally ≈0.3 mm) in an axial view of the iliac arteries (Figure 1). On arteriography (Figure 2), the stenosis was evident only when the arterial diameter of the proximal left iliac artery was compared with the contralateral iliac artery or the femoral segment. Surgery consisted of shortening the artery, resection of the fibrotic lesions, and enlargement of the artery with a saphenous vein patch. Figure 3 shows the histological findings of severe stenosis (≈75% of the vessel lumen) on cross section of the artery. Three years after surgery, the patient is asymptomatic during exercise.