Trisomy 13 mosaicism occurs when two cell lines, one with a normal complement of chromosomes and the other with an additional chromosome 13, are present in the same individual. We present two children with trisomy 13 mosaicism and summarize the literature in 47 published cases. Our first patient is a 4-year-old male with normal development and dysmorphic features that include right microtia, tall forehead, bulbous nose, and high arched palate. The second patient died at 1 day of life secondary to laryngeal stenosis, a previously undocumented finding in trisomy 13. His other dysmorphic features included micrognathia, cleft soft palate, and an axillary pterygium. Several published case reports of patients with trisomy 13 mosaicism have had the typical phenotype of complete trisomy 13 with death in the neonatal period, while others have had few dysmorphic features and prolonged survival. The most common malformations in all 49 cases included ear anomalies, cleft lip and palate, and various congenital heart defects. Intelligence varied from normal in six patients to significant delays and mental retardation in the remainder of cases. There is no clear correlation between the percentage of trisomic cells and the level of intellectual function. The dysmorphic features seen vary considerably from one patient to the next, often making the clinical diagnosis of trisomy 13 mosaicism difficult. In counseling families, medical professionals should state that trisomy 13 mosaicism may lead to physical abnormalities and poor intellectual outcomes, but that the condition does not do so universally with normal development occurring in some individuals. © 2009 Wiley-Liss, Inc.