Type of study: Case report. Setting: Department of Gynecology and Obstetrics, Department of Radiology, Department of Pathology, Department of Medical Genetics, Charles University Hospital, Hradec Králové. Methods: The autors report a case of renal tumor in a fetus at 33 weeks of gestation detected by means of ultrasonography and magnetic resonance imaging. The pregnancy course was complicated by polyhydramnios. The female infant was born vaginally at 37 weeks of gestation. Postnatal nephrectomy confirmed prenatal diagnostic presumption – congenital mezoblastic nephroma. At folllow-up at 12 months of age, the infant had no evidence of the disease. Conclusion. Detection of renal tumor in a fetus is rare. Congenital mezoblastic nephroma with the favourable prognosis is most probable diagnosis. Prenatal detection enables the prevention of complications (prematurity prevention, detection of the developing cardial decompensation and fetal hydrops) with the planning of labour and postnatal management.

        Key words: mezoblastic nephroma, ultrasonography, magnetic resonance imaging, ring sign