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ntroduction: Video assisted thoracic surgery (VATS) is a new minimal invasive diagnostic and therapeutic method in lung diseases that provides a perfect view of pleura and pericardium to perform a diagnostic biopsy from the pathologic site. This study aimed to evaluate the diagnostic value of VATS in pleural effusion with an unknown cause.
 
 
Materials and Methods: In this descriptive study, carried out from 1999 to 2007 in thoracic surgery department of Ghaem Hospital, sixty five patients with undiagnosed exudative pleural effusion underwent VATS for tissue diagnosis, and six months follow up was performed. Age and sex, pathology and diagnostic accuracy rate collected by questionnaire and processed using the SPSS software by descriptive statistics and frequency distribution table.
 
Results: Among 65 patients, 33.8% were female and 66.2% were male. Mean age was 58.90±14.57 years. The most common clinical symptom was dyspnea which was seen in 87.7%. Pleural effusion was clear yellow in 55.4% and bloody in 44.6%. Malignant cytology was reported only in one case (1.5%). Transthoracic needle biopsy showed pleuritis in 86.2%, fibrosis in 10.8% and it was suspicious in 3.1%. VATS was diagnostic in 95.4% and exact diagnosis wasn’t obtained only in 3 cases (4.6%). Open biopsy was performed in these cases that indicated Mesotelioma in two cases, another case with the history of CABG was treated after 6 months of follow up. The most common diagnosis was metastatic carcinoma, which observed in 37 cases (56.9%).VATS diagnostic accuracy rate in this study was 95.4%. No complication was seen in 98.5% and only one case had bleeding (1.5%). Mortality rate, because of VATS complications, was not seen.
 
Conclusion: According to the safety, high sensitivity and specificity of VATS, it is suggested as a good diagnostic method in pleural effusion with an unknown cause.