Purpose: This study describes the design of an ongoing randomized trial intended to determine which of two strategies is superior for managing small abdominal aortic aneurysms (AAA). Methods: Patients aged 50 to 79 years with AAA 4.0 to 5.4 cm in diameter as determined by computed tomography (CT) who are not at high surgical risk are randomized to either repair of the AAA, called "immediate surgery," or follow-up of the AAA with ultrasonography or CT every 6 months, reserving surgery for those aneurysms that enlarge to 5.5 cm, enlarge rapidly, or become symptomatic, called "selective surgery.". Results: The primary outcome measure is all-cause death, and secondary outcome measures are AAA-related death, morbidity, and general health status. The study design calls for 1350 patients to be randomized and monitored for a mean of 5 years. A second objective of the study is to accurately define the prevalence and risk factors for AAA with use of information from the large screening program established to detect AAA for recruitment into the randomized trial. Conclusions: By the end of 1993, 38,697 patients had been screened with ultrasonography, accounting for about three fourths of new randomizations, and 330 patients had been enrolled (70% of the target rate).