The authors report the unusual case of a 25-year-old man with occipitocervical pain related to a lesion of the C-1 lateral mass. Initially this lesion measured 8 mm and exhibited radiological features of an osteoid osteoma. Seven years later, as pain increased and became unresponsive to antiinflammatory drugs, computerized tomography scanning demonstrated progression to a 16-mm lesion, highly suspicious of an osteoblastoma. After mobilization of the vertebral artery from the C-1 groove, the lesion was completely resected via an anterolateral approach. Complete symptomatic relief, restoration of cervical range of motion and preservation of cervical stability were achieved immediately after surgery, and the results were confirmed at the 4-year follow-up examination. Pathological examination of tissue samples confirmed the diagnosis of osteoblastoma. Osteoid osteoma rarely evolves to osteoblastoma. Deterioration of a patient's ability to control pain is a warning sign. Insight into such cases underlines the importance of close long-term radiological follow-up examination in patients with conservatively treated osteoid osteomas.