L ymphopenia in an otherwise healthy person is typically discovered when doing routine blood counts. In Denmark, patients with incidental lymphopenia are usually not referred for further examination because the mortality implications are unknown. In general, predictors of mortality are highly valued in everyday clinical practice because they help identify patients who may benefit from additional medical attention. Observational studies have shown lymphopenia to be associated with risk of cardiovascular disease, 1-5 cancer, 6-11 liver disease (including alcohol abuse disorders) 12,13 and systemic autoimmune disease. 14-16 In a 2018 study involving 98 344 participants from the general population of Denmark, we found that lymphopenia was associated with a high risk of infectious disease. 17 The risk of all-cause mortality for patients with incidental lymphopenia is not known. This study aimed to identify the associations between lymphopenia and all-cause and cause-specific mortality. Methods We followed the reporting standards outlined in the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines. 18 Study design and participants Participants who were white, of Danish descent and aged 20-100 years were eligible if they participated in the Copenhagen General Population Study, 17 a prospective cohort study that enrolled participants from November 2003 to April 2015 from suburbs in and around Copenhagen, Denmark. 19-21 Further details can be found in Appendix 1, Supplementary Methods,