Mosaic trisomy 18 occurs when two different cell lines exist in the same individual; one cell line has two copies of chromosome 18, while the other has three copies. Here we present two new patients with mosaic trisomy 18, summarize 31 reported cases from the literature, and discuss management and counseling themes. Our first patient is an 8½-year-old female with normal intelligence and no significant dysmorphic features other than short stature and cubitus valgus. The second patient is a 21-month-old male with developmental delay, several dysmorphic features, including a patent ductus arteriosus, and normal growth. In general, the phenotype of individuals with mosaic trisomy 18 varies greatly. Some individuals have the complete trisomy 18, Edwards syndrome phenotype with early death while others are phenotypically completely normal. The latter group is exemplified by four normal appearing adults with mosaic trisomy 18 who were identified only after giving birth to children with complete trisomy 18. Further, a wide range of anomalies have been reported, most at low frequencies, including microcephaly, delayed bone age, brachydactyly, congenital heart defects, developmental delay, short stature, and premature ovarian failure. Intellectual capabilities range from profound mental retardation to above average intelligence. There appears to be no correlation with the percentage of trisomic cells in either fibroblasts or leukocytes and the individual's phenotype or intellectual function. We also discuss a variety of counseling issues including long-term survival, reproductive capacity of individuals with mosaic trisomy 18, and recurrence risks. © 2007 Wiley-Liss, Inc.