May-Thurner syndrome (MTS) is an uncommon clinical entity that requires treatment to prevent complications.A 45-year-old man presented to the emergency department with a one-week history of pain in the quadriceps muscle of the left lower limb, which had worsened over the previous 24 hours, accompanied by swelling and violaceous discoloration. He reported a 12-hour car- trip two weeks previously, at symptom onset. His medical history was notable for a sister who had three deep vein thromboses (DVT). Complementary tests showed: – Blood Test: D-dimer 4,900 ng/mL. – Thoracic CT-scan: Bilateral Pulmonary Embolism (PE). – Left leg ultrasound: Extensive thrombosis of the common, superficial, deep femoral veins, popliteal vein and tibioperoneal trunk. Subcutaneous oedema. The initial diagnosis was left femoropopliteal DVT with secondary bilateral PE. He began intravenous anticoagulation at the hospital. Oral anticoagulation and follow-up assessment post-discharge, including an abdominal ultrasound to rule out cancer. Two weeks later, he returned due to fever and increased pain: antibiotics initiated for possible cellulitis. After two months, patient received a final diagnosis of bilateral PE and MTS with associated DVT. Phlebography was performed, followed by venous revascularization and iliac stenting.Among the differential diagnoses, scientific literature highlights: Phlegmasia cerulea dolens, Phlegmasia alba dolens, DVT, Aortoiliac Aneurysm, Cellulitis, Osteoarthritis, Gout, Heart failure, Abdominal cancer, Metastases and Pelvic diseases (Klein, 2024; Mousa, 2025). MTS is very uncommon, particularly in men. This case could easily have been missed due to patient being an atypical candidate.This case expands the range of conditions to rule out when DVT is suspected. Detailed anamnesis and additional diagnostics are advisable to determine aetiology and prevent complications.In MTS, the left common iliac vein is compressed between the spine and the right common iliac artery, causing venous obstruction and thrombus formation. Phlebography is the diagnostic gold standard while treatment involves thrombectomy and endovascular stenting.When DVT is suspected, MTS may be present. This pathology should be considered due to its potentially complicated course and the need for specific treatment. In cases of DVT, underlying causes may range from simple triggers to anatomical abnormalities. Entity may appear in primary care.