Purpose: Dealing patients with organophosphate poisoning, cholinesterase level has been used as a diagnostic and prognostic value. But there are some controversies that the cholinesterase level is significantly related to the severity or prognosis of acute organophosphate poisoning. We evaluated the correlation between initial serum level of cholinesterase and APACHE II score as an index for severity, and we assessed cholinesterase levels for predicting value of weaning from mechanical ventilation. Method: From August 1996 to March 2003, 23 patients with organophosphate poisoning who needed ventilatory care were enrolled. Retrospective review was done for the serum level of cholinesterase, APACHE II score, and the duration of ventilatory care. The percentage of measured serum cholinesterase to median normal value was used to standardize cholinesterase levels from different laboratories. Result: There were tendencies that the lower initial serum of cholinesterase, the higher the APACHE II score (r=0.297) and the longer the duration of mechanical ventilation (r=-0.204), but they were not significant (p=0.264 and p=0.351 respectively). In 9 patients whose serum cholinesterase level were checked at the time of weaning, mean of measured cholinesterase level was $10.3pm7.60\%$ of normal value. Conclusion: There was no significant relationship between initial level of serum cholinesterase and severity or duration of mechanical ventilation. General health status of patient, amount of ingestion, toxicity of agent should be considered as important factors for severity of poisoning. And the decision of weaning should be based not solely on the cholinesterase level but on the consideration of general and respiratory state of individual patients.