This prospective cohort study aimed to clarify differences in the longitudinal effects on oral health-related quality of life (OHRQoL) among patients undergoing endodontic treatment for irreversible pulpitis and for pulp necrosis, using a newly developed oral health-related endodontic patient's quality of life (OHQE) scale. This study included 131 patients diagnosed with irreversible pulpitis and pulp necrosis. Comprehensive data regarding the patient's background, medical history, and dental history were collected. The OHQE was administered three times to each patient: before and after endodontic treatment, as well as 2 weeks after endodontic treatment as a follow-up. Statistical analysis was performed using a linear mixed model for repeated measurements of changes in the OHQE score over time in cases of irreversible pulpitis and pulp necrosis. The patients consisted of 48 (36.6%) males and 83 (63.4%) females with a mean age of 36.2 (standard deviation, 12.6) years. Of these, 62 (47.3%) had irreversible pulpitis, and 69 (52.7%) had pulp necrosis. Intragroup comparisons showed an improvement in the OHQE scores over time in both groups (p < 0.001). Group comparisons revealed no significant differences at any time point. No interactions or changes over time were observed between the two groups. No difference in the improvement of quality of life after endodontic treatment was seen in the two disease groups, and both groups improved over time. However, patients' expectations of receiving endodontic treatment remained unchanged after treatment. Therefore, dental providers should consider explaining the value of endodontic treatment to patients and address the measures that contribute to patient satisfaction.