The asymmetry of tonsillar ectopia, syringomyelia, and clinical manifestations, and their correlations in adult Chiari I malformation (CIM) are seldom discussed. Clinical and imaging data of 104 consecutive adult patients with CIM and syringomyelia were retrospectively reviewed. A method was devised to quantify tonsillar and syrinx asymmetry. Correlations between the asymmetrically displaced cerebellar tonsils, the side of the syrinx, clinical presentations, and the curve direction of scoliosis were investigated. Tonsillar ectopia was left dominant in 46 patients (44.2 %), right dominant in 49 (47.1 %), and symmetrical in nine (8.7 %). The syrinx was left deviated in 44 patients (42.3 %), right deviated in 48 (46.2 %), and centrally located in 12 (11.5 %). A significant correlation was observed between the side of tonsillar herniation and the side of the syrinx (p < 0.001), and also between the descending ratio of tonsillar herniation and the deviation ratio of the syrinx (p < 0.001). The main side the of clinical presentations showed significant correlation with the dominant side of tonsillar herniation (p = 0.009) and the side of syrinx deviation (p = 0.012). In the 49 patients (47.1 %) with associated scoliosis, the curve direction was significantly related to not only the dominant side of tonsillar ectopia (p = 0.0,28) but also the deviated side of the syrinx (p = 0.044). Moreover, the curve magnitude was significantly correlated with the distance of the tonsillar herniation (p = 0.001). In adult CIM, most tonsillar herniations are asymmetrical and most syringomyelia is eccentrical. We speculate that the dominant side of tonsillar herniation determines the side of syrinx deviation, which in turn determines the main side of clinical presentations and the convex side of scoliosis. Our results suggest that the more the descended tonsil tilts to one side, the more the syrinx tilts to the same side.