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Background The natural history of clinically stable patients with Chiari I malformation (CM-I)-syringomyelia is uncertain. To understand their outcomes, we examined conservatively managed CM-I-syringomyelia patients’ long-term clinical and radiological courses.Methods We enrolled 156 mild CM-I-syringomyelia cases (Japanese Orthopaedic Association (JOA) score ≥13) managed non-surgically between 1994 and 2014 and followed them periodically until December 2024 for significant progressive myelopathy that we termed ‘obvious deterioration’. Obvious deterioration was defined as a ≥2-point decline in JOA score to less than 13. Spontaneous syrinx resolution was radiologically defined as >50% reduction in syrinx length or maximal axial diameter on T1-weighted MRI.Results The entire cohort had over 1401 patient-years of follow-up. 55 patients exhibited clinical deterioration, yielding an annual progression rate of 3.9%. Obstructive sleep apnoea-hypopnoea syndrome (OSAHS) (HR=1.841, 95% CI 0.999 to 3.392; p=0.049), positive Babinski sign (HR=2.252, 95% CI 1.229 to 4.125; p=0.009) and without spontaneous resolution (HR=20.308, 95% CI 4.804 to 85.849; p<0.001) independently predicted later clinical obvious deterioration. Spontaneous resolution of CM-I-syringomyelia was more frequent with cervical syringes (HR=2.12, 95% CI 1.224 to 3.674; p=0.007) and absence of OSAHS (HR=3.83, 95% CI 1.376 to 10.640; p=0.01).Conclusion This study showed that the natural course of myelopathy in CM-I-syringomyelia varies according to the OASHS status, Babinski sign and spontaneous syrinx resolution. Additionally, baseline characteristics, including the spinal region of the syrinx and the absence of OSAHS, correlated with spontaneous syrinx resolution.