PURPOSE: To compare and characterize distinctive features of the involvement pattern of hypertrophic olivary degeneration, focusing on its various causes and the extent of hemorrhage. MATERIALS AND METHODS: In 15 patients (M:F=10:5; age range, 39-66 years) with hypertrophic olivary degeneration, the MR imaging findings were retrospectively reviewed. Signal intensity and changes in the size of the olivary nucleus were recorded, focusing on the causes of the condition and the extent of hemorrhage and involvement, as seen on T2-, proton density-weighted, and FLAIR images. The findings of follow-up study were available in three cases. RESULTS: In three patients with substantial hemorrhaging in the central portion of the pons or midbrain, bilateral hypertrophic olivary degeneration was observed, and in one with a large infarction in the pons, bilateral degeneration was revealed by MR images obtained three months later. In nine cases of unilateral hemorrhage of the pons or midbrain, unilateral hypertrophic olivary degeneration was present, and in one case of unilateral hemorrhage in the dentate nucleus, degeneration was contralateral. In one case of central hemorrhage in the pons and midbrain, bilateral degeneration was noted initially, and a fuzzy margin and increased high signal intensity of the olivary nucleus were noted on T2-weighted images obtained at follow-up nine months later. Contrast enhanced T1-weighted images depicted no definite enhancement. CONCLUSION: Hypertrophic olivary degeneration arises as secondary change due to phenomena such as hemorrhage or infarction, and shows variable patterns according to its causes.