This case report describes a man with C1-C2 cervical myelopathy due to an os odontoideum. Over a span of 30 years, the patient developed weakness of his left lower limb, which gradually spread to all four limbs. The diagnosis was not made for three decades despite several medical consultations, a myelogram, and a computed tomography scan of the brain. This case illustrates that neurological disease due to spinal chord compression can have a long history, and demonstrates how advances in magnetic resonance neuroimaging contribute to a definite and accurate diagnosis.