Objectives: This study defines a protocol for monitoring the growth of small abdominal aortic aneurysms based on age-related abnormality thresholds, observed aortic growth patterns and the limits of precision of the measurement technique. Design: 13000 men aged 60 – 75 years were invited to their GP's surgery for measurement of the maximum diameter of the infrarenal aorta. Seventy-six percent responded and measurement was possible in 97.1% of the respondents. The AP diameter of the aorta was measured according to a strict protocol, with a portable ultrasound scanner. Setting: Cases were recruited from the conurbation of Birmingham and Solihull, U.K. Chief outcome measure: A statistical description of the differences in distributions of aortic diameters of four age groups of males. Main result: The aorta expands in diameter during the seventh and eighth decade for up to 25% of all those screened. The order of magnitude of this change is similar to that previously attributed to the growth of small aneurysms. Age-related thresholds for abnormality should range from 24 mm for a 60 year old to 37 mm for a 75 year old. Conclusions: Using data on the expected maximum rate of change of the dilated aorta and statistically derived thresholds from this analysis a monitoring strategy is suggested for those with an abnormal aorta.