The significance of Intraoperative ultrasound (US) monitoring during foramen magnum decompression (FMD) was studied in five patients. Two of the patients had Chiari type I malformation with syringomyelia, one had Chiari type I malformation with hydrocephalus, and two had stenosis of the foramen magnum (Chiari type I malformation with basilar compression and achondroplasia) . Intraoperative ultrasound examinations were performed with a linear array real-time unit (ALOKA Corporation, Japan) equipped with a 7.0-MHz transducer. In four of the patients, re-establishment of cerebrospinal fluid flow was clearly demonstrated. One patient with hydrocephalus revealed adhesion at the foramen of Magendie. Neurological symptoms and signs improved in four of the five patients and were unchanged in one patient with obstructive hydrocephalus. The most important ultrasound findings during Intraoperative us monitoring were clear demonstration of the subarachnoid space at the cisterna magna, pulsation of the cerebellar tonsil, and cerebrospinal fluid flow in the foramen of Magendie. We conclude from this study that Intraoperative us monitoring during FMD is necessary for following the clinical progress.