Background: The magnitude of overdiagnosis is a critical and unresolved issue in lung cancer (LC) screening:(1) its contribution to the increase in survival constitutes specious evidence of benefit;(2) overdiagnosed individuals who undergo resection will experience a reduction in life expectancy, partially or completely offsetting the benefit received by others in whom earlier intervention proves curative. Method: Critical analysis of studies in opposition and support of the view that LC screening imposes a substantial burden of overdiagnosis. Results: Approximately 25%, possibly more, of radiographically (chest x ray) diagnosed LC appears to be overdiagnosed. Based on the observed tumour volume doubling time of low dose CT identified small malignant pulmonary nodules, CT will markedly augment lead time, increasing exposure to competing lethal morbidities, thereby increasing overdiagnosis. Conclusion: To reduce all-cause mortality, CT screening will need to reduce LC mortality by an amount that exceeds the increase in mortality attributable to surgery and loss of pulmonary reserve in persons who are overdiagnosed or pathologically understaged (ie, with occult micrometastases). Presently, there is no evidence that CT screening will achieve any reduction in LC mortality.