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Magnetic resonance imaging (MRI) study on degeneration of the thoracic spine in asymptomatic subjects.To investigate the incidence of degenerative MRI findings of the thoracic spine in asymptomatic subjects and to identify factors related to the degeneration of the thoracic discs.Studies on age-related degenerative changes of the thoracic spine are scarce.Ninety-four asymptomatic Japanese volunteers (48 men and 46 women, mean age of 48.0 ± 13.4 years) underwent MRI of the thoracic and cervical spine and filled the questionnaire regarding life styles. The items evaluated on MRI using a numerical grading system were (1) decrease in the signal intensity of the intervertebral discs (DSI), (2) posterior disc protrusion (PDP), (3) anterior compression of the dural sac (ACD), and (4) disc space narrowing. Association between each degenerative MRI finding and several factors, including age, sex, smoking, sports, body mass index, and degeneration of cervical spine was investigated.Forty-four (46.8%) patients demonstrated positive degenerative MRI findings at 1 or more thoracic intervertebral levels. The percentage of the subjects with positive MRI findings was 37.2% in DSI, 30.9% in PDP, 29.8% in ACD, and 4.3% in disc space narrowing. The percentages of all MRI findings increased with aging. In 85 (90.4%) patients, degenerative MRI findings were positive in the cervical spine. DSI was significantly associated with age (odds ratio, 11.21, 95% confidence interval, 2.70–46.5), PDP with age (3.44, 1.02–16.61), smoking (4.94, 1.55–15.71) and presence of PDP in the cervical spine (4.25, 1.01–17.92), and ACD was associated with smoking (3.99, 1.28–12.44).Degenerative changes in the thoracic spine on MRI was observed in approximately half of the asymptomatic subjects, whereas their incidences were less frequent than those in the cervical spine. Factors significantly associated with degenerative changes in the thoracic spine included age, smoking, and degeneration in the cervical spine.