We report an unusual cause of unilateral lymphedema of the right limb rapidly increasing in a young woman. Ultrasonography ruled out the diagnosis of iliac deep venous thrombosis or extrinsic compression: the B mode scan revealed a mass located below the aortic bifurcation and along the iliac vessels, without any compressive effect. The lesion was heterogeneous associating both tissular and cystic aspects. The lower limb lymphoscintigraphy showed an interruption of the colloid circulation at the right iliac level. Computed tomography did not give any additional information. Magnetic resonance imaging before then after gadolinium showed typical aspects of cystic lymphangioma and confirmed the ultrasonography hypothesis. Considering that this tumor is benign, that surgery would be difficult because of the anatomic situation of the mass, and that post-operative involution of the edema is uncertain, the treatment was based on compressive stockings and regular follow up.