The authors report on a case of trisomy 9 mosaicism syndrome, a rare chromosome abnormality. The common features of this syndrome are growth and mental retardation, low-set malformed ears, wide sutures and fontanelles, bulbous nose, short palpebral fissures, micrognathia, microphthalmia and enophthalmos, abnormal hands and feet, hip dislocation, joint limitation, cardiovascular defects and urogenital abnormalities. Our patient presented some unusual characteristics, such as 13 pairs of ribs, a vertebral malformation, a hemivertebra and a Dandy-Walker syndrome. They compare their clinical findings with the few cases previously described and they try to contribute to the further clinical definition of the syndrome. It is possible that there is a correlation between the variability of the phenotype and the percentage of trisomic cells in the patient.