Trisomy 16 is the most common trisomy in spontaneous abortions and is usually, if not always, lethal in the nonmosaic state. We report a liveborn infant with trisomy 16 mosaicism first diagnosed by amniocentesis at 20 weeks gestation. At birth, the infant was growth retarded and mildly dysmorphic. At age 14 months she was developmentally normal and had facial asymmetry. Her length, weight and head circumference were normal. Pure trisomy 16 was found in cells from the placenta. A normal female karyotype was found in lymphocytes from the infant. Skin fibroblasts revealed a trisomy 16 karyotype in 6 of 30 cells. Molecular analysis showed maternal uniparental heterodisomy, indicating that the trisomic conceptus arose from a nondisjunction of maternal meiosis. Fibroblasts may be the tissue of choice for detection of low-level trisomy 16 mosaicism.