OBJECTIVE To delineate the natural history of infrarenal aortic aneurysms with respect to growth rate. METHODS A referral based series of 233 patients with abdominal aortic aneurysm was serially assessed by abdominal ultrasound. RESULTS The mean age was 71 years and the median follow up 26 months (range 6-146). The mean growth rate of the series was 3.2mm/year. Division of the initial size into those less than 41 mm (128 patients), between 41-60 mm (92 patients) and those above 60 mm (13 patients) in AP diameter showed mean growth rates per year were 2.6 mm/year, 4.1 mm/year and 6.5 mm/year. Examination of the relationship between aneurysm size and subsequent growth rate over the whole study period revealed a rank correlation of 0.19 (p = 0.002). Aneurysms were also observed to expand at an erratic rate both in terms of time and size. CONCLUSIONS When the initial decision is to observe the patient, serial ultrasound examinations facilitate later judgements on the balance of risks in relation to surgical treatment.