This is the report of two independent families in which a balanced maternal translocation led to trisomy 12 p in one of each their offspring. Evaluation of 21 further case reports indicates that this is a phenotypically well defined syndrome which leads to severe developmental retardation. It can be recognized by a characteristic combination of craniofacial anomalies which are summarized in a phantom picture. The gene sequences which produce the typical features in the trisomic state must be localized distally to band 12p12, which is the breakpoint in the partial trisomies. The specific craniofacial anomalies are not visibly modified by the length of the trisomic segment or additional small monosomies or trisomies of recipient chromosomes. However, the frequency and severity of organ malformations and the resulting probability of survival seem to decrease with increasing degrees of chromosomal imbalance. A cytogenetic classification of the 21 inherited translocations and a segregation analysis from the pedigree data was performed. For the different types of translocations the calculated risk figures are given.