AbstractExcessive Ca intakes have been proposed to associate with vascular calcification and a higher risk of prostate cancer. We investigated the associations of supplemental and dietary Ca intake with mortality using data from 497 828 UK Biobank participants. The average follow-up was 4·2 years and 14 255 participants died, 8297 from cancer, 2959 from CVD and 572 from respiratory disease. The use of Ca supplements and milk consumption were associated with differences in mortality in younger (≤65 years) but not in older participants (>65 years, P_interaction ≤ 0·04 for all comparisons). Among participants <65 years, there was an inverse association between Ca supplementation (OR 0·91, 95 % CI 0·83, 0·99) and milk consumption (OR 0·93, 95 % CI 0·86, 1·00) with respect to all-cause mortality. In the same age group, milk drinkers had lower odds of cancer mortality (OR 0·89, 95 % CI 0·80, 0·98) but Ca supplement use was associated with increased odds of respiratory mortality (OR 1·69, 95 % CI 1·16, 2·74). All associations in participants aged ≥65 years were null after full adjustment. In sensitivity analyses stratified by hormone replacement therapy, Ca supplement use was associated with decreased odds of cancer mortality in users but increased risk in other women (OR 0·81, 95 % CI 0·69, 0·94 v. OR 1·17, 95 % CI 1·01, 1·35, respectively). To conclude, we saw little evidence for harm with dietary or supplemental Ca. Further studies are required to confirm the proposed interaction with hormone replacement therapy and to exclude reverse causation as a determinant in the association between Ca supplements and increased risk of respiratory diseases.