Introduction: Suicidal poisoning with organophosphorus (OP) pesticides is common, particularly from rural areas. This highlights the importance of determining an OP poisoning prognosis to decide how aggressive treatment should be. There are reports suggesting a relationship between prolonged corrected QT (QTC) interval and the severity of poisoning. We aimed to evaluate the prognostic utility of this clinical tool in OP poisoning (OPP) patients.Methods: Patients with the primary diagnosis of OPP who were admitted to the intensive care unit (ICU) of Loghman-Hakim Hospital Poison Centre (LHHPC) were the subjects of this prospective study. Cholinesterase (CE) activity and the QTC interval was determined for each patient using the Bazett formula and considering <440 msec as normal. Comparative outcomes of the study were duration of both hospitalization and mechanical ventilation, serum CE activity on admission and its daily level, total amount of atropine administered, analysis of the QT and QTC intervals in the primary ECG on admission and at the end of hospitalization, and rate of mortality.Results: The study included 42 patients with a diagnosis of OPP. The mean age of the patients was 32, ranged from 12 to 81 years old. The mortality rate was 37.5%. There was no significant difference between two groups (prolonged and normal QTC intervals) according to gender and age (p = .491 and p = .133, respectively). The CE level for long and normal QTC interval groups was 3.90 Ϯ 0.33 kU/L vs. 4.41 Ϯ 0.23 kU/L, respectively. The mortality rate in the long QTC group was significantly higher than that of the normal QTC group (p = .044). Moreover, the average period of hospitalization in patients with prolonged QTC interval was higher than the other group (p = .02). The average atropine required to control the muscarinic signs and symptoms such as salivation, bronchorrehea, and miosis in patients with prolonged QTC interval was 38.60 mg; in patients with normal QTC interval it was 20.02 mg (p = .013).Conclusion: QTC interval prolongation may have prognostic value in OPP.