Rationale: The ratio of neutrophil-to-lymphocyte (NLR), monocyte-to-lymphocyte (MLR) and platelet-to-lymphocyte (PLR) are biomarkers that have shown potential for predicting mortality in several diseases. For patients hospitalized with community-acquired pneumonia (CAP), the prognostic capabilities of these biomarkers are unknown. Objective: Investigate whether NLR, MLR or PLR were associated with 90-day mortality in CAP. Further, investigate whether the prediction rule CURB-65 could be improved by adding these biomarkers. Methods: A derivation-validation study using a Danish multicentre retrospective cohort as the derivation cohort (N = 831) and a European multicentre prospective cohort as the validation cohort (N = 2463). Associations between biomarkers and mortality were assessed using Cox proportional hazard models with adjustments for sex, CURB-65 and comorbidities. A cut-off value for biomarkers was determined using Youden’s J Statistics. The performance of CURB-65 with added biomarkers was evaluated using receiver-operating characteristics. Results: In both cohorts increasing NLR and PLR were associated with 90-day mortality. In the derivation cohort, the hazard ratios for NLR and PLR were 1.016 (95% confidence interval (CI) 1.001–1.032, P = 0.038) and 1.001 (95% CI 1.000–1.001, P = 0.035), respectively. Adding these biomarkers to CURB-65 did not improve its performance. Conclusions: NLR and PLR were associated with 90-day mortality in CAP, but did not improve CURB-65.