We report on a 20-month-old male, diagnosed prenatally with de novo mosaic ring chromosome 18 and low level monosomy 18, who also exhibited an inherited and apparently balanced translocation between chromosomes 3 and 6. We believe this to be the first reported case of prenatally diagnosed mosaic ring chromosome 18 and monosomy 18 in which the child was carried to term. Ring chromosomes are associated with an abnormal phenotype that is dependent on the amount of material that is deleted from the p and q arms. This child has a 22.5 Mb deletion of 18q and a 2.8 Mb deletion of 18p as a result of ring formation. Although the large deletion has resulted in some developmental delays and health problems, the child is making more developmental progress than was anticipated prenatally. We present his clinical course and the genetic counseling challenges associated with this case.