Duplication of the internal jugular vein (IJV) is a rare malformation. Three intraoperative cases are reported. In our personal experience, the clinical incidence of the anomaly is approximately 4 per 1,000 unilateral neck dissections. The venous duplication is at a variable height, affecting the superior part of the IJV. The lateral branch of the accessory nerve (XI) always passes medially to the anterior vein and laterally to the posterior vein, between the venous duplication. This is most often unilateral but sometimes bilateral. The IJV may be normal, dilated or ectatic. The discovery of this anatomical variation has practical implications during cervical lymph node clearance, either functional or radical, during oncological surgery necessitating viewing the IJV and its affluents and the lateral branch of the accessory nerve. The embryological explanation suggests a topographical "conflict" between the development of the IJV and the lateral branch of the accessory nerve. The French version of this article is available in the form of electronic supplementary material and can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00276-002-0020-y.