Regional distribution of cerebral blood flow was determined semi-quantitatively with 99Tcm-HMPAO brain SPET under basal conditions in Type 1 (insulin-dependent) diabetic patients of recent onset and longer disease duration, and related to metabolic control and history of hypoglycaemic events. Long-term diabetic patients showed significantly more alterations in regional cerebral blood flow than diabetics of recent onset and healthy controls. Regional hypoperfusion, predominantly localized in the fronto-temporal cortex, was almost exclusively observed in patients with long-term diabetes. The latter finding was related to lower HbA1c levels (i.e. better metabolic control) and to the frequency of impending hypoglycaemia, but not to age of the patient, duration of diabetes or to chronic diabetes complications. The incidence of hypoperfusion was comparable in patient groups with or without a medical history of hypoglycaemic coma. However, regions of hypoperfusion were larger in the patients who had experienced hypoglycaemic coma. It is concluded that regional cerebral hypoperfusion in long-term Type 1 (insulin-dependent) diabetics, as evidenced by HMPAO-SPET can be related to the frequency and degree of hypoglycaemic events and to tight metabolic control, which is however at the expense of an increased risk of recurrent hypoglycaemia.