This study examines the rate of expansion of abdominal aortic aneurysms and the risk of rupture in relation to their size. To assess these variables, we conducted a prospective study of 300 consecutive patients who presented over a 6-year interval with abdominal aortic aneurysms (AAA) that were initially managed nonoperatively. The mean age of the patients was 70.4 years, and 211 (70%) were men. The mean initial aneurysm diameter was 4.1 cm. Among the 208 patients who underwent more than one ultrasound or computed tomographic (CT) scan, the diameter of the aneurysm increased by a median of 0.3 cm per year. The 6-year cumulative incidence of rupture was 1% and 2% among patients with aneurysms less than 4.0 cm and 4.0 to 4.9 cm in diameter, respectively (p greater than 0.05). In comparison, the 6-year cumulative incidence of rupture was 20% among patients with aneurysms greater than 5.0 cm in diameter (p less than 0.004). We conclude that (1) abdominal aortic aneurysms expand at a median rate of 0.3 cm per year; and (2) the risk of rupture of abdominal aortic aneurysms less than 5.0 cm is substantially lower than the risk of rupture of aneurysms 5.0 cm or more in diameter.