Thoracic aortic aneurysms (TAAs) are potentially lethal medical conditions often requiring surgical intervention. Reliable information on TAA growth rates and associated risk factors is important for managing this challenging patient population. Unfortunately, a number of studies have employed questionable statistical methods, leading to biased and imprecise estimates. The present study describes these statistical problems in existing studies and delineates procedures for obtaining more reliable results. Using data from the Yale Center for Thoracic Aortic Disease, the study compares TAA growth rate estimates using conventional methods versus the recommended approach of instrumental variables (IV) estimation. The IV approach is designed to mitigate problems of measurement errors inherent in existing estimates of TAA growth. The results demonstrate that IV estimation yields more robust and precise estimates of TAA growth rates and risk factors for TAA growth. For example, the conventional approach yields TAA growth rates that fluctuate substantially-from 0.12 cm/yr to 0.90 cm/yr-depending on (1) the minimum serial follow-up period for patient inclusion in the study and (2) how subjects with negative measured growth rates are handled. In contrast, growth rate estimates using the IV approach are much more robust, ranging from 0.12 to 0.13 cm/yr. The 95% confidence intervals of estimated TAA growth are much more compact using the IV approach as well. We conclude that the IV estimation procedure yields more reliable estimates of TAA growth than does the conventional approach.