Abstract： Objective To investigate the incidence of pleural effusion in acute panereatitis(AP)and the relation between pleural effusion and severity of AP.Methods The medical records of 766 patients with AP were analyzed retrospectively.The incidence of pleural effusion was documented and its relationship with age of onset,sex,Ranson score,CTSI,serum albumin and hospital day was analyzed.Results In the 766 AP patients,there were 129(16.8%)patients had pleural effusion.In 171 SAP patients,there were 91 (15.3%)patients had pleural effusion.In the 595 MAP patients,there were 129(16.8%)patients had pleural effusion.The difference in the incidence of pleural effusion between MAP and SAP patients was statistically significant(P＜0.05).The serum amylase,albumin,Ranson score,CTSI,and hospital day in patients with pleural effusion were(795±1013)U/L,(36±7)g/L,1.12±1.15,4.02±1.16 and(23.4±23.4)d,respectively;while the corresponding values in patients without pleural effusion were(592±856)U/L,(38±6)g/L,0.85±0.98,3.15±16.60 and(17.3±16.6)d,respectively.The difference in Ranson score,serum amylase,CTSI between the two groups was statistically significantly(P＜0.05 or＜0.01＝,the serum level of albumin in patients with pleural effusion was significantly lower than that in patients without pleural effusion(P＜0.01),the hospital day in patients with pleural effusion was significantly longer than that in patients without pleural effusion(P＜0.01).Pleural effusion of both sides was present in 92 patients,while pleural effusion of right side was present in 11 patients,pleural effusion of left side was present in 26 patients;the site of pleural effusion was not associated with the severity of AP,however,patients with pleural effusion of left side tended to have longer hospital day(P＜0.05).Conclusions Patients with pleural effusion during AP were more likely to be complicated with severe conditions,and the presence of pleural effusion may be a good marker for severity evaluation.