The aim of the study was to correlate between the clinical score described by Peradenya Organophosphorus Poisoning (POP) scale, serum cholinesterase level at presentation and severity of poisoning. Consecutive patients of organophosphorous poisoning attending Bir Hospital from August 2004 to September 2005 were studied. Patients were grouped into mild, moderate and severe poisoning groups according to the POP scale. Fifty patients fulfilled the inclusion criteria. The severity of poisoning directly correlated with serum cholinesterase level (P<0.001). The differences in the mean requirements of atropine on the first day of admission, in the total amount of atropine needed to treat and in the average duration of hospital stay were significant (P<0.05). There were 26% patients in moderate poisoning and only 4% patients in severe poisoning, but a total of 14% of the patients died, indicating that patients with even moderate degree of poisoning had also died. Both the POP scale severity and derangement in serum cholinesterase levels at initial presentation did not correlate with mortality, which could be due to various other co-morbidities and inadvertent stoppage of atropine infusion particularly at night in the wards. The POP scale and serum cholinesterase at presentation appeared useful to assess the severity of poisoning, particularly in terms of higher amount of atropine and prolonged duration of hospital stay. The patients with evidence of moderate and severe degrees of poisoning need close monitoring. Further studies to evaluate the factors likely to cause deaths, particularly by closely monitoring of moderate and severe organophosphorous poisoning in intensive care units, are required to clarify the correlation with mortality.