In 2013, the US Preventive Services Task Force (USPSTF) recommended low-dose computed tomographic (CT) screening for high-risk current and former smokers with a B recommendation (indicating a level of certainty that it offered moderate to substantial net benefit). Under the Affordable Care Act, the USPSTF recommendation requires commercial insurers to fully cover low-dose CT. The Centers for Medicare & Medicaid Services (CMS) is now considering whether to also offer coverage for Medicare beneficiaries. Although the National Lung Screening Trial (NLST) demonstrated the efficacy of low-dose CT, implementation of national screening may be premature. The magnitude of benefit from routine screening is uncertain; estimates are based on data from a single study and simulation models commissioned by the USPSTF. The potential harms-which could affect a large population-include false-positive results, anxiety, radiation exposure, diagnostic workups, and the resulting complications. It is unclear if routine screening would result in net benefit or net harm. The NLST may not be generalizable to a national screening program for the Medicare age group because 73% of NLST participants were younger than 65 years. Moreover, screening outside of trial conditions is less likely to be restricted to high-risk smokers and qualified imaging centers with responsible referral protocols. Until better data are available for older adults who are screened in ordinary (nontrial) community settings, CMS should postpone coverage of low-dose CT screening for Medicare beneficiaries.