Objective: This study was performed for the determination of the expansion rates and outcomes and for recommendations for the surveillance of the 3.0-cm to 3.9-cm abdominal aortic aneurysm (AAA). Design: The study was observational with data from patients screened with ultrasound scanning for AAA at five Veterans Affairs Medical Centers for enrollment in the Aneurysm Detection and Management Trial. The eligibility requirements included: AAA from 3.0 cm to 3.9 cm in diameter and at least one repeat ultrasound scan more than 90 days after the initial screening. Patients also completed a questionnaire for demographic data and the determination of the presence of risk factors associated with AAA. The study endpoints included: 1, both mean and median expansion rates; 2, moderate expansion (>4 mm/year); 3, no expansion; 4, all causes of death; 5, AAA rupture; 6, expansion to 4 cm or more; 7, expansion to 5.0 cm or more; and 8, operative repair. Results: Ultrasound scan screening results identified 1445 patients with 3.0-cm to 3.9-cm AAAs. Seven hundred ninety men met the ultrasound scan criterion of having at least two ultrasound scan studies during the study period, and these 790 men were used for this study. Mean AAA size was 3.3 cm, with an average follow-up period of 3.89 ± 1.93 years. The median expansion rate was 0.11 cm/year. Expansion rates were significantly different (P < .001) between 3.0-cm and 3.4-cm cm AAA and 3.5-cm and 3.9-cm AAA. There were no reported AAA ruptures during the study period, although cause of death data were available in only 43% of the patients. Few 3.0-cm to 3.9-cm AAAs expanded to 5.0 cm or more during the study period. The patients with 3.0-cm to 3.9-cm AAAs who underwent operative repair during the study period were younger, had larger initial AAA diameters, and had more rapid expansion rates. Conclusion: AAAs of 3.0 cm to 3.9 cm expanded slowly, did not rupture, and rarely had operative repair or expanded to more than 5.0 cm in our study of male patients. Expansion rates and the incidence rate of operative repair are more common in the 3.5-cm to 3.9-cm AAA when compared with the 3.0-cm to 3.4-cm AAA.