In three patients, two males aged four months and 39 years, and a female aged 19 years, trisomy 9p (the threefold presence of the short arm of chromosome 9) was diagnosed. The main symptoms are mental retardation, brachycephaly, hypertelorism with deep-set eyes, broad base of the nose, short philtrum, carp-shaped mouth, cup-shaped ears, short fingers and clinodactyly. Trisomy 9p is often caused by a balanced translocation in one of the parents. Therefore, cytogenetic studies of the parents and if appropriate other relatives are necessary. If one of the parents carries a translocation, the recurrence risk may vary from 2% to 15%. This depends on the length and origin of the chromosomal translocation segments. In case a translocation is found, genetic counselling of family members is warranted; it should include information concerning clinical symptoms, recurrence risks, prenatal diagnosis and other family planning alternatives.