Red cell distribution width (RDW) is a prognostic factor in many diseases; however, its clinical utility remains limited because the relative value of RDW as a biomarker across disease states has not been established. To establish an unbiased RDW disease hierarchy to guide the clinical use of RDW and to assess its relationship to cardiovascular hemodynamic and structural parameters. We performed phenome-wide association studies for RDW in discovery and replication cohorts derived from a deidentified electronic health record in nonanemic individuals. RDW values obtained within 30 days of echocardiogram or right heart catheterization were tested for association with structural and hemodynamic variables. RDW was associated with 263 phenotypes in both men and women in the discovery cohort ( = 121,530), 48 of which replicated in an independent cohort ( = 2,039). The strongest associations were observed with pulmonary arterial hypertension (odds ratio [OR], 2.1; 95% confidence interval [CI], 1.9-2.3), chronic pulmonary heart disease (OR, 2.0; 95% CI, 1.9-2.2), and congestive heart failure (OR, 1.9; 95% CI, 1.8-2.0);