Self-reported tobacco pack-year history plays a large role in decisions about low-dose computed tomography (CT) lung cancer screening yet is challenging to measure accurately.To what extent does random measurement error in pack-year information impact screening decisions and screening effectiveness?and Methods: Retrospective cohort study of 10,449 patients with pack-year history documented at least twice between 10/2013-7/2017, across 8 academic Veterans Affairs sites.1) Observed reliability of pack-year information based on all repeat measures for the study population; 2) Each person's statistically "true" pack-year information based on best linear unbiased predictor from a multilevel linear random effects model. To examine how unreliability leads to misclassification of screening eligibility and inaccuracy in estimating lung cancer risk, we simulated pack-year observations for each person, first comparing simulated pack-year and lung-cancer-risk values with "true" values, then comparing outcomes when basing screening decisions on unreliable pack-year information vs. "true" information.Reliability of assessing pack-year information in routine practice varied across sites. Thus, we examined the clinical impact of two different levels of reliability, based on the range of ICCs observed. Using a > 30 pack-year threshold led to a high rate of eligibility misclassifications (48.1% misclassified with higher reliability pack-year information and 60.7% with lower reliability information). However, using a lung cancer risk threshold leads to fewer misclassifications (47.3-49.7% misclassified when using lower reliability pack-year information) and better maintains screening effectiveness when using unreliable pack-year information.Random error in real-world pack-year assessments leads to a substantial rate of misclassifying who should be offered CT screening if a > 30 pack-year criterion is used. However, using a lung cancer risk threshold mitigates the impact of unreliable pack-year information. Decision-makers concerned about the impact of unreliable pack year information should consider using risk-based approaches to CT screening.