BACKGROUND: Although iliac vein thrombus is uncommon in adolescents, it can present with pelvic inflammatory disease (PID) symptoms. CASE: A 19-year-old sexually active female with Klippel-Trénaunay syndrome (KTS) presented with fever, abdominal and lower extremity pain. Physical findings included cervical motion tenderness and left lower extremity swelling and erythema. The patient was admitted for PID and cellulitis. Despite antimicrobial treatment her pain continued. Neisseriae gonorrhea and Chlamydia trachomatis cultures were negative. Abdominal and pelvic computed tomography scans revealed a left internal iliac vein thrombus. Anticoagulation therapy was initiated; her pain improved. On hospital day 7 she developed pulmonary emboli. SUMMARY AND CONCLUSION: In sexually active adolescents with known risk factors for thromboembolism such as KTS, symptoms and signs considered characteristic for PID can be present in association with an iliac vein thrombus.