A case control study of transient global amnesia (TGA), transient ischaemic attacks (TIA) and normal controls is described. Each of the 51 TGA patients, selected between January 1985 and March 1990, was compared with four controls (two TIAs and two normals) for the presence of vascular risk factors (hypertension, diabetes, smoking habits, cholesterol, triglycerides and haematocrit levels, heart disease, previous stroke), previous TGA, migraine, psychiatric illness and recent head trauma. Patients with TGA had less diabetes, hypercholesterolaemia and hypertriglyceridaemia than TIA. TGA subjects had significantly more hypertension (odds ratio = 3.31) and migraine (odds ratio = 8.67) than normal controls. During a mean of 17.4 mths of follow-up (range 1-96 mths), three subjects had recurrent TGA, one sustained a TIA and a minor stroke, but none had seizures. Thrombo-embolism and epilepsy are unlikely to be the cause of this benign disorder. The role is stressed of appropriate precipitants, including haemodynamic changes, and of individual susceptibility (of which migraine is probably a marker) in the genesis of TGA.