A mosaic trisomy 13 presenting as a case of cleft lip and palate in the newborn is described. When the child was admitted to the hospital at the age of 6 wk because of failure to gain weight and a malformation of the great vessels was demonstrated, cytogenetic studies were carried out. The diagnosis of mosaic trisomy 13 (90% normal, 10% trisomic) was established from a leukocyte culture. Since mosaic trisomy 13 may occasionally mimic cleft lip and palate in the newborn, cytogenetic studies are indicated in the presence of any additional anomaly.