Aim — the objective of the present article was to report the clinical case of detection of the embologenic gigantic i gantic floating thrombus in the inferior vena cava by non-contrast enhanced magnetic resonance phlebography (MRP) in an outpatient setting. Material and methods. A 65-year-old woman applied for the medical assistance in connection with the swelling of the right lower extremity that appeared 5 days earlier. She complained of oedema, bursting pains, and sensation of heaviness in the affected limb. The patient had a malignant cerebral tumor. Ultrasound duplex scanning revealed total thrombosis of the entire deep venous bed of the right lower. The upper part of the thrombus was impossible to identify due to poor visualization of the veins of the retroperitoneal space. Non-contrast enhanced magnetic resonance was performed immediately on an outpatient basis. Results. A gigantic floating thrombus 24 cm in length was identified in the inferior vena cava, with its upper part located in the infra-renal area. The point of fixation of the thrombus was located in the vena iliaca externa. Its base was thinned and the width at the site of attachment was 1.5 mm. Conclusion. The clinical case reported in this article demonstrates the possibility of the application of non-contrast enhanced magnetic resonance phlebography for the visualization of the inferior vena cava and its main tributaries above the inguinal fold under the outpatient conditions and its advantages over other methods for the same purpose. The MRP technique does not require a special preparation of the patient for hospitalization in the case of suspected venous thrombosis.