Antibiotic use has been associated with an increased risk of cancer in epidemiologic studies. We evaluated the association between antibiotic use and the risk of primary lung cancer by conducting a prospective casecontrol study nested in a cohort of subjects who were 40 to 84 years old in 1995 to 2004, with at least 2 years of enrollment in The Health Improvement Network. There were 4,336 cases of primary lung cancer. A random sample of 10,000 controls was frequency matched to the cases for

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... sex, and calendar year of diagnosis. Antibiotic exposure was measured by the total number of antibiotic prescriptions and the cumulative number of days on antibiotics since enrollment. We discounted exposure 1 year before the date of cancer diagnosis. Relative risks (RR) and 95% confidence intervals (95% CI) were estimated using conditional logistic regres- Materials and Methods We conducted a nested case-control study within a population-based cohort using computerized medical record information from the THIN database in the United Kingdom (11, 12) . THIN collects data on around Cancer Epidemiol Biomarkers Prev