A woman, aged 31, had nausea, vomiting, slurred speech, and diplopia after vigorous manipulation of the neck during chiropractic treatment. There was weakness of the left arm and leg and decreased sensation for touch and pallesthesia over the left half of the body, including the face. The patient's underlying disease was believed to be multiple sclerosis, but the acute episode represented a brain-stem vascular accident secondary to manipulation of the neck. A man, aged 55, had a similar treatment twice. Signs and symptoms became worse with each manipulation. Examination revealed Horner's syndrome on the left side, paralysis of the left vocal cord, and diminished sensation of pain and temperature over the right half of the body. Manipulation of the neck can significantly narrow the lumen of the vertebral artery, and susceptibility to such accidents may depend on an abnormal vascular supply to the basilar arterial system.