There have been several published definitions for abdominal aortic aneurysm (AAA), however, it is not known how different definitions affect what is reported. This paper presents an evaluation of three definitions taken from the literature on (a) aneurysm frequency, (b) prevalence rate and (c) prevalence ratio from an ongoing case-control study evaluating whether male siblings of patients with known AAA have a higher prevalence of aneurysms compared to a control group. Depending on the definition used, the frequency of AAA could vary from a low of five to a high of 23; the prevalence of AAA in siblings could range from 2.1 to 18.8. Changing definitions alters the prevalence ratio (risk of disease) in both magnitude and direction. On the one hand, male sibling of affected patients are at approximately one and a half times (1.6) greater risk of having an AAA compared to controls, whereas using a different definition they are at less than half the risk (0.3).