Aims: It is believed that arthrodesis of spinal segments leads to excessive stress at unfused adjacent levels. The incidence, prevalence and radiographic progression of symptomatic adjacent-segment disease were studied. Methods: A series of 85 patients who had an anterior cervical arthrodesis for the treatment of cervical spondylosis with radiculopathy or mielopathy were reviewed with a minimum of 7 months and a maximum of ten year follow-up. The annual incidence of adjacent-segment disease was defined as the percentage of patients who had been disease-free at the start of a given year of follow-up in whom new disease developed during that year. The prevalence was defined as the percentage of all patients in whom symptomatic adjacent-segment disease developed within a given period of follow-up. The hypothesis that new disease at an adjacent level is more likely to develop following multilevel arthrodesis was also tested. Results: Symptomatic adjacent-level disease occurred at an incidence of 5,3% per year, with an 18,2% maximum at the third year of follow-up and a final prevalence of 34,1%. The greatest risk of new disease was at the interspaces of the sixth and seventh cervical vertebrae. Patients with a multilevel arthodesis were significantly more likely to have symptomatic adjacent-level disease. Conclusions: Symptomatic adjacent-segment disease may affect more than one-third of all patients within ten years after an anterior cervical arthrodesis. A multilevel arthrodesis and the interspaces between the sixth and seventh cervical vertebrae appear to be the greatest risk factors for new disease...