A preterm girl was born at a gestational age of 30+4 weeks, with hydrops foetalis and accompanying ascites due to supraventricular tachycardia. Progression of the ascites was noted during an infection. Ultrasonographic evaluation revealed a cystic mass in the right abdomen and a right-sided hydronephrosis. Milky fluid, corresponding with parenteral nutrition, was obtained at ultrasound-guided aspiration of ascites. A contrast radiograph showed intra-abdominal extravasation from a central venous line inserted in the right lower limb. After removal of the central venous line both the cystic mass and the hydronephrosis disappeared. To our knowledge, retroperitoneal extravasation from a central venous line leading to hydronephrosis has not been reported before. Malposition with extravasation is a well-known complication of a central venous line. This may present in various ways, including the appearance of a cystic mass. In our case, this led to an obstruction of the urine flow and hydronephrosis. Malposition with extravasation should be considered when unexplained local signs and symptoms occur in any patient with a central venous line. Copyright © 2011, Nederlandse Vereniging voor Intensive Care.