Higher Red Blood Cell Distribution Width (RDW or anisocytosis) predicts incident coronary artery disease (CAD) plus all-cause and cardiovascular mortality, but its predictive value for other common diseases in healthy volunteers is less clear. We aimed to determine the shorter and longer term associations between RDW and incident common conditions in participants free of baseline disease, followed for 9 years. We undertook a prospective analysis of RDW% using 240,477 healthy UK Biobank study volunteers aged 40-70 years at baseline, with outcomes ascertained during follow-up (≤9 years). Participants were free of anemia, CAD, type-2 diabetes, stroke, hypertension, COPD, and any cancer (except non-melanoma skin cancer) at baseline. Survival models (with competing Hazards) tested associations with outcomes from hospital admission records and death certificates. High RDW (≥15% variation, n = 6,050) compared to low (<12.5% n = 20,844) was strongly associated with mortality (HR 3.10: 95% CI 2.57 to 3.74), adjusted for age, sex, smoking status, education level, mean cell volume and hemoglobin concentration. Higher RDW was also associated with incident CAD (sub-HR 1.67: 1.40 to 1.99), heart failure, peripheral vascular disease, atrial fibrillation, stroke, and cancer (sHR 1.37: 1.21 to 1.55; colorectal cancer sHR 1.92: 1.36 to 2.72), especially leukemia (sHR 2.85: 1.63 to 4.97). Associations showed dose-response relationships, and RDW had long-term predictive value (≥4.5 years after assessment) for the majority of outcomes, which were similar in younger and older persons. In conclusion, higher RDW predicted onsets of a wide range of common conditions as well as mortality in a large healthy volunteer cohort. RDW is not just a short term predictor, as high levels were predictive 4.5 to 9 years after baseline in healthy volunteers. The wide range of outcomes reflects known RDW genetic influences, including diverse disease risks. RDW may be a useful clinical marker for inclusion in wellness assessments.