This prospective study analyzes 100 patients with Chiari malformation and primary craniovertebral junction (CVJ) anomalies (3-66 years). Neurodiagnostic investigations employed tomography, gas myelography, CT and CT myelography, and MRI. Factors considered were reducibility, mode of encroachment, cerebrospinal fluid (CSF) dynamics and syringohydromyelia. Sixty-six patients with irreducible pathology underwent ventral or ventrolateral decompression and dorsal stabilization. Dorsal occipitocervical fixation was performed in reducible lesions that also required dorsal decompression (n = 34). Proatlas remnants were identified in 8 and atlas assimilation in 92 patients. Paramesial invagination was present in 20, syringohydromyelia in 46, and vertebral segmentation defects in 66 others. Completely reducible abnormalities were identified in 16 of 20 patients aged 2-14 years, and partially reducible abnormalities in 4 of 16 patients aged 14-20 years, 8 of 48 patients aged 20-40 years and 6 of 16 patients aged 40-60 years. The critical sagittal canal diameter at the foramen magnum was 19 mm. Twenty-two patients had previous posterior decompression and 27 had previous syrinx to subarachnoid shunt with delayed deterioration. Improvement occurred in all after ventral or ventrolateral decompression with resolution of the syringohydromyelia and normalization of CSF flow. We conclude that: (1) hindbrain herniation syndrome is frequently seen with fourth occipital sclerotome abnormalities; (2) Chiari malformation with craniovertebral abnormalities become symptomatic with a canal diameter of < 19 mm; (3) abnormal ventral bony pathology is reducible in children wit atlas assimilation and later becomes irreducible invagination, therefore early operation with fusion is recommended; (4) ventral decompression relieves brain stem, cerebellar symptoms and syringohydromyelia; (5) CSF studies with cine MRI shows reversal of craniospinal CSF dissociation after ventral CVJ decompression and; (6) craniovertebral anomalies associated with Chiari malformations must be addressed early and appropriately.