Background <p>Primary Chiari malformation type 1 (CM1) and type 0 (CM0) appear to share features of overcrowding of the posterior cranial fossa (PCF) neural elements. We carried out the present study to identify anatomical features differentiating primary Chiari malformation type 1 (CM1) from Chiari malformation type 0 (CM0) and normal subjects, and then used those anatomic features for CM1 and CM0 phenotyping and diagnosis.</p> Methods <p>This study included two groups of adult patients: CM0 with syringomyelia (SM) (<i>CM0-SM</i>) having cerebellar tonsillar descent ≤ 2&#xa0;mm below the foramen magnum (FM), CM1 with syringomyelia (<i>CM1-SM)</i> with tonsillar herniation (TH) &gt; 3&#xa0;mm, and normal controls. Clinical data and MRI parameters were analyzed.</p> Results <p>CM1 and CM0 associated with SM share anatomic characteristics of PCF hypoplasia, size reduction, and flattening, and share clinical findings of occipital headache and cervical central myelopathy. CM1 has more pronounced PCF hypoplasia, narrower FM CSF pathways, and more expansive and extensive syringes than CM0, while CM0 has reduced FM size. Combining two MRI morphometric markers predicted CM1 with a sensitivity of 94%, and three markers predicted CM0 with a sensitivity of 90%. In CM1 and CM0, the descent of the PCF neural structures, assessed by TH and obex descent relative to the FM, correlated with PCF flattening and FM size but not with PCF size.</p> Conclusion <p>Four PCF morphometric markers independent of TH can accurately predict the presence of CM0 and CM1 associated with syringomyelia.</p>