The purpose of this study is to examine the change of enzymeimmuno-assay for $prostaglandinE_2$ in the synovial fluid lavage specimen of patients with mandibular fracture patients without condylar fracture. For this study, fourteen patients (eight males, six females) with mandibular fractures without condylar fracture was investigated to analyse the synovial fluid from upper temporomandibular joint cavities. Synovial fluid was collected from TMJ cavities of mandibular fracture patients before open reduction and after one week of open reduction, and then stored in liquid nitrogen tank after centrifuge. Two synovial fluid lavage samples of TMJs of 2 asymptomatic served as normal controls referred from other data. The concentrations of $PGE_2$ were measured by use of $PGE_2$ EIA System ($Amersham^{(R)}$). The following results were obtained: 1) In nine patients, the concentrations of $PGE_2$ are lower after open reduction than before. 2) In three patients, the concentrations of $PGE_2$ are higher after open reduction than before. 3) There was no statistical significant between the preoperative group and postoperative 7 days (p>0.05), but there was some difference between the two groups. In conclusion, the results suggest that $PGE_2$ probably does not play as important role in the harm of TMJ.