Objectives: To define the natural history of ectatic abdominal aortas and to assess the clinical need for follow-up. Design: Abdominal aortas were considered ectatic if they were diffusely and irregularly dilated with a diameter less than 3 cm. Ectatic aortas were identified either by AAA screening or as incidental findings. Patients who had only one scan were excluded from the study. Clinical data were analysed. Setting: Two district general hospitals in Wales and England. Subjects: 116 patients (90 men). Results: The median age of patients was 71 years (range 48-90). Co-existing risk factors included hypertension (75), IHD (22), PVD (8), diabetes (3), COAD (14), stroke (5), popliteal aneurysm (1), malignant disease (3) and 4 had a family history of AAA. The median follow-up was 24 months (range 5-72). The median and maximum growth rate of the ectatic aortas were 0.65 and 14.4 mm/year respectively. In three patients the expansion rate was more than 5 mm/year. In 22 patients the ectatic aorta became aneurysmal, reaching a diameter greater than 3 cm. There were no ruptures and no elective repairs. Two deaths occurred due to IHD. Conclusions: This study demonstrates that if ectatic aortas do expand they do so very slowly. However, 22 of the 116 (19%) became aneurysmal in a follow-up of twp years. Once identified ectatic aortas should be scanned at intervals of three years. (C) 2003 Published by Elsevier Science Ltd on behalf of The International Society for Cardiovascular Surgery.