Aim To determine the probability of the incidence, intensity, duration and triggering of postendodontic pain, considering factors related to the patient (age, gender, medical evaluation) and to the affected tooth (group, location, number of canals, pulp vitality, preoperative pain, periapical radiolucencies, previous emergency access, presence of occlusal contacts with antagonist). Methodology A total of 500 one-visit root-canal treatments (RCTs) were performed to patients referred to an endodontist. Shaping of root canals was performed manually with Gates-Glidden drills and K-Flexofiles, and apical patency was maintained with a size 10 file. A 5% NaOCl solution was used for irrigation, and canals were filled with lateral compaction and AH-plus sealer. Independent factors were recorded during the treatment, and characteristics of postendodontic pain (incidence, intensity, type and duration) were later surveyed through questionnaires. Out of the 500 questionnaires, 374 were properly returned and split in two groups for two different statistical purposes: 316 cases were used to adjust the logistic regression models to predict each characteristic of postendodontic pain using predictive factors, and the remaining 58 cases were used to test the validity of each model. Results The predictive models showed that the incidence of postendodontic pain was significantly lower when the treated tooth was not a molar (p=0.003), demonstrated periapical radiolucencies (p=0.003), there was no history of previous pain (p=0.006) or emergency endodontic treatment (p=0.045) and there was no occlusal contact (p