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Background: At least 10% of lung cancers arise in adults who have never used tobacco. Data remain inconclusive on whether lung cancer incidence has been increasing among adults who have never used tobacco., Research Question: How have age-adjusted incidence rates of lung cancer changed temporally, especially among adults who have never used tobacco?, Study Design and Methods: Trends in lung cancer incidence were examined using linked electronic health record and cancer registry data on a dynamic cohort of adults ≥ 30 years of age at risk of incident lung cancer between January 1, 2007, and December 31, 2018, from an integrated health-care system in northern California. Truncated age-adjusted lung cancer incidence rates and average annual percentage change (AAPC) in rates were estimated, overall and separately for adults who have ever and never used tobacco by age, sex, and race or ethnicity., Results: The cohort included 3,751,348 adults (52.5% female, 48.0% non-Hispanic White, 63.1% have never used tobacco), among whom 18,627 (52.7% female, 68.6% non-Hispanic White, 15.4% have never used tobacco) received a diagnosis of lung cancer. The overall lung cancer incidence rate declined from 91.1 to 63.7 per 100,000 person-years between 2007 and 2009 and between 2016 and 2018 (AAPC, -3.9%; 95% CI, -4.2% to -3.6%). Among adults who have ever used tobacco, incidence rates declined overall from 167.0 to 113.4 per 100,000 person-years (AAPC, -4.2%; 95% CI, -4.4% to -3.9%) and, to varying degrees, within all age, sex, and racial or ethnic groups. Among adults who have never used tobacco, incidence rates were relatively constant, with 3-year-period estimates ranging from 19.9 to 22.6 per 100,000 person-years (AAPC, 0.9%; 95% CI, -0.3% to 2.1%). Incidence rates for adults who have never used tobacco seemed stable over time, within age, sex, and racial or ethnic groups, except for those of Asian and Pacific Islander (API) origin (AAPC, 2.0%; 95% CI, 0.1%-3.9%), whose rates were about twice as high compared with their counterparts., Interpretation: These observed trends underscore the need to elucidate further the cause of lung cancer in adults who have never used tobacco, including why incidence is higher and rising in API adults who have never used tobacco., (Copyright © 2023 American College of Chest Physicians. Published by Elsevier Inc. All rights reserved.)