We report a case of prenatally diagnosed mosaic trisomy 20 in cells cultured from amniotic fluid. Trisomy 20 was present in 7 cells (13 percent) in a total of 52 investigated cells. Following the normal findings of an ultrasound scan, the couple decided to continue the pregnancy. A dysmorphic infant was born at the 38 weeks of gestation with generalized dysmorphic features and multiple cardiac anomalies including transposition of great arteries. Chromosome analysis on both cord blood and placenta at birth revealed a normal 46,XX karyotype. This patient is the first case of a liveborn infant with mosaic trisomy 20 cells detected in amniotic fluid culture with transposition of great arteries, atrioventricular concordance and ventricoarterial discordance.