Abstract: Sonography was performed by two expert radiologists separately after selecting 80 patients (45 men and 35 women) whose pleural fluids had been aspirated and examined by the lab. The radiologists were given no clinical information concerning the patients, and the result compared with lab results. The radiologists evaluated three criteria in determining the nature of the pleural effusion: septation, echogenicity, and thickening of pleura by more than 3 mm. The study showed that the pleural effusion with septation or internal echogenicity is always an exudate. Also sonographic evidence of thickened pleura (more than 3 mm) is highly suggestive of an exudate. Although an anechoic effusion is more probably evidence of a transudate, we have seen it in 14% of patients with exudates. The lab results showed that there were 29 patients with transudates and 51 with exudates, and in ultrasound results there were 34 with transudates and 46 with exudates. A transudate is always without echogenicity, while exudates can be with or without echogenicity. It was therefore concluded that sonography is useful in determining the nature of pleural effusion.