Objective. To study the morbidity and mortality of patients with aneurysms of the posterior circulation (basilar-vertebral system) and investigate the natural history of these aneurysms without surgical intervention. Methods. Of 1232 patients with intracranial aneurysms admitted to Tiantan Hospital from January 1990 to October 2001, 153 (12.4%) had aneurysms of the posterior circulation. Their clinical and angiographic data were reviewed retrospectively. Additionally, to June 2002, 38 of the cases managed without surgery have been followed by telephone and correspondence. For these patients, the annual aneurysm rupture rate was estimated in "person-years". Results. Patients were aged 4-69 years (mean 40.8). The peak incidence was in the 5th decade. There were 88 males and 65 females. One hundred and two patients (66.7%) were treated surgically, and of these, 60 (58.8%) had an excellent outcome, 10 (9.8%) a good outcome, 29 (28.4%) a fair outcome and 3 (2.9%) died. The other 51 (33.3%) patients were treated conservatively for various reasons. Of these, 13 died of ruptured aneurysm in hospital. Ten of these patients had had a previous subarachnoid haemorrhage and 3 had not. The other 38 surviving, conservatively managed patients were followed for 5-95 months (mean 23). Of them, 3 (17.6%) died of aneurysmal rupture, giving an annual rupture rate of 9.2%, or 3/32.47 patient years. Conclusions. To prevent death and disability due to aneurysmal rupture, posterior circulation aneurysms should be treated aggressively. (C) 2004 Published by Elsevier Ltd.