Vrabl (1) reports the case of a 32-year old female patient with hereditary multiple exostoses, one of which affected the C5 arch and, in concurrence with chronic posttraumatic C4-6 instability, caused a 2/3 stenosis of the spinal canal. Hereditary multiple exostoses (osteochondromas), an autosomal dominant disorder, also called diaphyseal aclasis, is one of the more frequent hereditary diseases of the skeleton. We registered and are following this disturbance of skeletal development in 111 cases who received treatment between 1966 and 1995. The disease individually varies in degree from just a few, small, and hardly apparent exostoses, mostly on long bones of the extremities, to very severe forms with extensive involvement of the skeleton, severe deformities of the extremities, and often an involvement of the axial skeleton. There exists no treatment that is directed against the causes of the disease, surgical removal of all lesions is impossible, removal of osteochondromas is indicated in large lesions limiting mobility, in case of compression of important structures (nerves, vessels), or if malignant change is suspected. Malignant transformation, mostly into chondrosarcoma, is rather frequent; different authors reported between 10 and 20%. In our series, malignant change occurred in 10 patients. Osteochondromas are extremely rarely located in the vertebrae; until 1995, 38 cases of osteochondromas were reported in the cervical spine (22 solitary exostoses and 16 multiple ones). The posterior parts of the vertebrae are more often affected. Removal is indicated in all osteochondromas invading the spinal canal before they cause serious compression of the spinal cord, especially in hereditary exostoses, in which growth activity of the lesions is uncertain. In the case Vrabl reported, the exostosis affected the C5 arch and exerted pressure on the C6 arch as well causing its deformation. In addition, the patient had a skiing accident that caused segmental instability of C4-C6, and damaged the intervertebral discs C4-5 and C5-6. Osteophytes developed at the level of the damaged intervertebral discs. The extent of pathological findings was exactly assessed by imaging methods: CT, MRI and discography. The double affection of the cervical spine caused a slow progression of tetraparetic symptoms and was a clear indication for surgical intervention. The exact diagnostic evaluation revealed the two etiologic factors of spinal cord compression and was of great importance for treatment. It would have been a serious mistake to concentrate only on the more obvious osteochondroma of C5. A biphasic operation with a 3 weeks' interval was performed. First the C5 osteochondroma and part of the deformed C6 arch were posteriorly removed. The degenerated discs C4-5 and C5-6 were excised anteriorly 3 weeks later, and foraminotomy and spondy!odesis of the unstable segments were carried out. The postoperative course was uneventful, and the neurological findings gradua1Iy no~-maiized. The author decided to perform the biphasic surgery in two sessions and not in one, with an initial posterior approach and an anterior approach after a time interval of 3 weeks. Even though the patient had to be operated on twice within three weeks, the chosen alternative seems to be preferable as there is a lower risk of additional damage to the spinal cord during surgery. Treatment results can be considered as excellent in this complicated case.