OBJECTIVES The aim of this study was to evaluate the effectiveness of Glasgow Coma Scale (GCS), Acute Physiology And Chronic Health Evaluation (APACHE-II) and Poison Severity Scores (PSS) to predict clinical outcome of organophosphorus (OP) poisoning in a south Indian hospital METHODS A prospective study of 100 acute OP poisoning patients who were admitted to the emergency centre of a tertiary care hospital in South India between January 2007 to February 2008 were enrolled in the study. All the demographical and laboratory data of the patient were recorded in patient profile forms. Severity of poisoning was assessed by using clinical severity indices like GCS, APACHE-II and PSS within 24 hours after admission. The correlations between these severity scores with prehospitalization period, cholinesterase levels, need for ventilation, atropine dose required, incidence of pneumonia, hospitalization days, mortality and were evaluated RESULTS A total of one hundred patients were evaluated for clinical severity at admission. The mean GCS, APACHE II and PSS values of the study population were 11.5 ± 4.21, 10.1 ±12.49 and 3.1 ± 0.46 respectively. Among study population 40% of the patients had GCS score less than 8 (sever) and 15% of patients had GCS score 9-12 (moderate) and 45% had a GCS score between 12-15(mild). Similarly 25% of patients had a PSS score of 3 (severely poisoned) and 45% had PSS score of 2 (moderately poisoned) and 30% had PSS score of 1(mild poisoning). Mechanical ventilation was required by 42% of patients who had a GCS of less than 8. CONCLUSIONS The clinical severity scores correlated significantly with cholinesterase level, total dose and duration of atropine and pralidoxime therapy, length of ICU stay and ventilator requirement and mortality and it was found to be statistically significant