Fifty patients with abdominal aortic aneurysms from 2.5 to 5.0 cm in anteroposterior diameter (median 3.1 cm, mean +/- S.E. 3.3 +/- 0.1 cm) were initially offered non-operative treatment. Two patients have subsequently undergone successful elective aneurysm resection because of increase in aneurysm size, and a third has died. The median annual growth rate of the aneurysm has been 0.22 cm and 77.8% increased in size between 6 monthly ultrasound examination. For aneurysms less than 4.0 cm the maximum 6 monthly increment in diameter was 0.7 cm. Even the smallest abdominal aortic aneurysms usually progressively increase in size and 6 monthly ultrasound remeasurement of aneurysm diameter is an essential component of non-operative management.