To assess the accuracy of death certificates in assigning lung cancer as the underlying cause of death, death certificate data were compared to mortality review committee-determined causes of death among participants in the Mayo Lung Project. Further, the impact of death certificate misclassification on lung cancer mortality rates and Cox proportional hazards models was evaluated. The Mayo Lung Project (1971-1983) was a randomized controlled trial of lung cancer screening; participants were male smokers aged 45 years and older who were seen as outpatients at the Mayo Clinic in Rochester, Minnesota. Overall there were 237 lung cancer deaths according to mortality review, and 224 according to the death certificate (sensitivity 88.6 percent, 95 percent confidence interval (CI) 83.9, 92.4; specificity 99.1 percent, 95 percent CI 98.6, 99.5). As compared to the mortality review committee's determination, the use of death certificate data resulted only in slight decreases to the calculated lung cancer mortality rates for each screening arm, and did not result in appreciable changes to hazard ratios for lung cancer mortality in Cox regression models. In these data, death certificates were sufficiently sensitive and specific such that their use did not result in a meaningful change to mortality-based outcomes.