Early assessment of patients presented with acute organophosphorus toxicity in the Emergency Department is an essential step to detect their pathway in the hospital. The aim of the present study was to assess the overall accuracy of Acute Physiology and Chronic Health Evaluation (APACHE II), Modifies Early Warning Score (MEWS) and Worthing Physiological Score (WPS) in predicting mechanical ventilation in acute organophosphorus toxicity patients. A cross sectional cohort study was conducted on 132 patients presented with acute organophosphorus patients. From each patient, the following was collected: clinical data, routine laboratory investigations results and blood samples for estimation of cholinesterase levels. The clinical and laboratory data were used to calculate APACHE II, MEWS and WPS scores within six hours from admission. Statistical analysis revealed that the median values of APACHE II, MEWS and WPS differed significantly between mechanically ventilated and non-mechanically ventilated patients. Strong significant negative correlation was detected between clinical severity and cholinesterase levels. WPS score showed the highest discriminatory power for predicting mechanical ventilation (area under the curve [AUC] 0.977). However, APACHE II and MEWS scores were nearly equal in their discriminatory power (AUC 0.924 and 0.927 respectively). Cholinesterase levels can be used as useful diagnostic tool but very poor in predicting patient outcome (AUC 0.209 and 0.129 for acetylcholinesterase and butyrylcholinesterase respectively). In conclusion, using WPS score for clinical evaluation of acute organophosphorus toxicity patients has valuable prognostic abilities for predicting patients' outcome.