While the clinical features associated with full trisomy 13 have been well characterized, the clinical outcome associated with mosaic trisomy 13 is much less clear. The medical literature reports a broad range of possible clinical outcomes from severe mental retardation and birth defects to normal intelligence. There is no consensus about the typical phenotype in these cases. This makes genetic counselling after prenatal diagnosis of mosaic trisomy 13 particularly difficult. Some of the medical literature attempts to correlate the percentage of trisomic cells in peripheral blood leukocytes or skin fibroblasts with clinical outcome. There have not been case reports correlating the percentage of trisomic amniocytes and clinical outcome. We report the prenatal diagnosis of mosaic trisomy 13 by amniocentesis in which no prenatal ultrasound abnormalities were noted, and autopsy was normal with the exception of the presence of a small ventricular septal defect. © 1998 John Wiley & Sons, Ltd.