Twenty-three cases of aortic dissection in patients with giant-cell arteritis are reviewed and an additional case is reported. Forty-six percent presented catastrophically with aortic dissection and no prior history of giant cell arteritis. Eighty percent died within 2 weeks of the event; four patients had successful surgical grafts. There was diffuse involvement of the aorta with giant cells in 89%, but dissecting tears occurred primarily in the proximal aorta in 85% of cases. The majority of cases with a preceding history of giant cell arteritis were on low doses of steroid or on no treatment at the time of dissection, and the median erythrocyte sedimentation rate of these patients was 62 mm/h (range 21-98). Evidence of some form of hypertension, whether acute or chronic, mild or severe, was found in 77% of patients. Inadequate treatment of giant-cell arteritis and underlying hypertension (treated or untreated) are potential factors leading to aortic dissection in these patients.