Dysthyroid optic neuropathy (DON) was diagnosed in 36 eyes of 21 patients with progressive visual loss and congestive ophthalmopathy. Systemic features in the patients with DON did not differ from those reported for Graves' disease patients except that patients with DON were older (mean age, 61 years) and did not show female preponderance. Congestive symptoms always preceded visual loss, which was gradual in onset and bilateral in most patients but acute and asymmetrical in several. Presenting acuities were poorer than 20/60 in 50% of cases; central scotomas, sometimes combined with inferior depression, were the predominant field defects. Congestive signs were of moderate intensity without severe proptosis or exposure keratopathy. Bilateral and symmetrical ductional restriction was the most common motility disturbance. Oral corticosteroids were effective in restoring visual function in ten of 21 eyes treated. Many steroid-unresponsive eyes were improved promptly by supervoltage orbital irradiation or surgical decompression. In general, therapeutic intervention appeared to hasten recovery and improve visual outcome.