Outcome assessment of endodontic treatment is critical for appropriate case selection and treatment planning. However, reports on outcomes of nonsurgical endodontic treatment vary considerably. Epidemiological studies done in a large patient population and over a long follow-up period can provide the clinician with useful tools for clinical decision-making and assessment of tooth prognosis. In this study, outcomes of initial endodontic treatment done in 1,462,936 teeth of 1,126,288 patients from 50 states across the USA was assessed over a period of 8 yr. Treatment was done by private general practitioners and endodontists participating in the Delta Dental Insurance plan that insures approximately 14 million individuals in the USA. Overall, 97% of teeth were retained in the oral cavity 8 yr after initial nonsurgical endodontic treatment. The combined incidence of untoward events such as retreatments, apical surgeries, and extractions was 3% and occurred mostly within 3 yr from completion of treatment. Analysis of the extracted teeth revealed that 85% had no full coronal coverage. A significant difference was found between covered and noncovered teeth for all tooth groups tested (p < 0.001). In conclusion, it appears that initial nonsurgical endodontic treatment is a predictable procedure with high incidence of tooth retention after 8 yr.