We present the first case of an infant with paternally-derived mosaic trisomy 16. Amniocentesis following an elevated maternal serum alpha-fetoprotein level and early fetal growth restriction at 19 weeks detected a high level of mosaicism with 25/33 colonies demonstrating trisomy 16 and 8/33 colonies with a normal 46,XX karyotype. Molecular studies revealed a paternal origin of the trisomy which was present in amniotic fluid cells, representing either a post-zygotic error or a meiosis II non-disjunction without crossing-over. In addition, there was normal biparental inheritance in the normal cell line. The symmetrically growth-restricted fetus was closely monitored for the remainder of the gestation. Decreased fetal movements at 36 weeks in conjunction with electronic fetal monitoring showing evidence of fetal distress necessitated abdominal delivery. Severe growth restriction, mild facial dysmorphism, and cardiac anomalies were identified. Microsatellite analysis demonstrated biparental inheritance in skin fibroblasts with a paternal origin for the trisomy in the placenta. Follow-up cytogenetic studies of additional tissues revealed 85 per cent trisomy 16 mosaicism in the placenta, yet only cytogenetically normal cells in lymphocytes and fibroblasts.