Lumbar back pain in an adolescent is generally musculoskeletal, that is, due to a muscle strain or scoliosis. This case describes a young individual without any previous health issues who rapidly developed a life-threatening condition, though initially presenting with "only" back pain. A 16-year-old Caucasian male patient was admitted twice to the emergency department with debilitating lumbar pain without neurological or vascular symptoms. Imagery showed an extensive thrombosis of the inferior vena cava. No external cause, structural abnormality, or any systemic disease were found that predisposed the patient to this highly unusual vaso-occlusive incident. Thrombosis of the inferior vena cava is a rare but life-threatening entity. It is underrecognized and associated with serious short- and long-term morbidity and mortality. Increased awareness is essential because deep vein thrombosis in children seems to cause atypical symptoms, such as spinal pain or the absence of edema of the lower limbs, as in the present case.