Objective. Chiari malformation is characterized by herniation of the cerebellar tonsils into the foramen magnum, which leads to disturbance of CSF circulation through the craniovertebral junction. Orthostatic stress, which leads to the movement of SCF through the craniovertebral junction, is an adequate method to detect these disorders. It is accompanied by changes in the intracranial pressure, affecting the cerebrovenous orthostatic reactivity (CVOR), which is noninvasively assessed in patients with Chiari malformation. Material and methods. The study involved 35 patients with Chiari malformation (26 patients with Chiari I and 9 patients with Chiari II) aged 4 to 58 years (of them 12 males). Hydrocephalus was diagnosed in 4 examined patients and myelosyringosis was diagnosed in 6 patients. Transcranial Doppler sonography was used to record the venous blood flow in the tentorial sinus of the brain while changing body position on the fracture table from +90° to –30°. Results. There is significant CVOR abnormality in most patients with Chiari malformation (more than 90%), which is characterized by either increased CVOR (sometimes 5—6-fold compared to the upper normal level (considerable hyperreactivity) or complete absence of any changes during the orthostatic load (areactivity). Before surgical treatment, CVOR of patients with Chiari malformation is often characterized by areactivity, as well as a moderate or significant hyperreactivity. After surgical treatment (decompression of the foramen magnum), patients with Chiari malformation demonstrate significant normalization of the craniovertebral volumetric ratios and CVOR if often characterized by normoreactivity (in 63%) or, more rarely, moderate hyperreactivity. The rate of venous blood flow in the tentorial sinus of the brain in patients with Chiari malformation can be increased before the surgery and normalizes after surgery. Conclusion. The high incidence of disturbance of CVOR (over 90%) in patients with Chiari malformation was revealed. After surgical treatment, complete normalization of CVOR was observed in more than half of these patients (63%).