Factors determining the outcome for patients with abdominal aortic aneurysm (AAA) were analysed in a retrospective population-based study of 187 consecutively diagnosed AAAs at one hospital during a 9-year period. All aneurysms were diagnosed by ultrasound, and those cases that were not primarily operated upon, were followed by repeat ultrasound examinations. An expansion rate of more than 0.4 cm/year was seen in 27% of the aneurysms and a tendency towards a higher rate of expansion could be seen with larger lesions. The overall cumulative rupture rate was 12% at 5 years. For patients with small (less than 5 cm) aneurysms it was 2.5% at 7 years, and no aneurysm could definitively be shown to be smaller than 5 cm at the time of rupture. The rupture risk was significantly higher (28% at 3 years) for larger aneurysms (greater than or equal to 5 cm). The only reliable predictor for rupture was aneurysm size. The overall cumulative survival was 51% at 5 years. Patients with large aneurysms did not have a significantly shorter survival although a tendency for this to be the case was found. There was a significant difference between the proportion of deaths caused by aneurysm rupture in patients with small aneurysms when compared to those with large aneurysms, 5.5 and 53%, respectively. The expansion rate for AAA was highly individual and aneurysm diameter was the only recognisable predictor of rupture. The rupture rate for AAAs smaller than 5 cm was lower than previously reported.