Fourteen patients with bilateral internal carotid artery thrombosis were analysed with respect to long-term prognosis and the prevalence of risk factors of cerebrovascular disease. The patients comprised seven per cent of all patients with internal carotid thrombosis treated in 1966-73 at the Department of Neurology, University of Helsinki. During the follow-up period of 3 to 86 months (median 50 months) five patients died, three during the first, and two during the second year of follow-up. Of the surviving patients: two needed institutionalization; three were able to take care of themselves; and four who were virtually symptom-free, were able to work. The risk factors of cerebrovascular disease were analysed: 91 per cent were cigarette smokers; 70 per cent had serum triglycerides of 140 mg/100ml or higher; and 62 per cent had cholesterol values of 280 mg/100 ml or higher. Fasting glucose values higher than 95 mg/100 ml, and ECG abnormalities were encountered in every third, and obesity and hypertension in every fourth patient. Cigarette smoking and high triglycerides were more prevalent than in the general population. No association between the number of risk factors in the individual patients and the prognosis could be found.