A 21-year-old girl with classical Prader-Willi Syndrome was found to have a 14;15 Robertsonian translocation--45,XX,t(14;15)(p11;q11). This type of Robertsonian translocation was not found in any patient from 8 surveys covering 6144 patients with mental retardation. Chromosome 15 has been involved in translocations in patients with the Prader-Willi Syndrome with greater than expected frequency. This is the first report of a 14;15 translocation and the Prader-Willi Syndrome. The same balanced translocation was present in the patient's mother and 2 normal siblings. Future genetic counselling for these 2 siblings will be difficult.