Triphasic delta waves on electroencephalography described in a variety of disorders1(pp38-39) are thought to be most characteristic of hepatic encephalopathy. Experience with a recently treated patient emphasizes that hyperthyroidism may also be associated with this pattern. The abnormality resolved as the patient responded to appropriate treatment. REPORT OF A CASE A 63-year-old man was well until January 1978, when he experienced the gradual onset of numbness and weakness of the left arm and leg. Examination disclosed weakness and increased tone in the left upper and lower extremities. There was hypalgesia below the umbilicus on the right side. Reflexes were hyperactive. There were clonus of the left ankle and a Babinski's sign on this side. Myelography revealed a large spur pressing on the cord at the C5-6 level. He underwent discectomy with anterior cervical fusion. The neurological findings persisted. Mental status was normal at discharge, but over the