Abstract: Objective To evaluate the role of ¹⁸F-FDG PET/CT in differential diagnosis of hydrothorax. Methods PET/CT was performed for 59 patients（43 males,16 females） during August 2007-June 2009 in our department due to unexplained hydrothorax which could not be definitely diagnosed by routine chest CT.Malignant hydrothorax was confirmed by pathological examinations,including tissue biopsy,pleural biopsy,and cytological examination of exfoliated cells in hydrothorax.PET/CT image analysis showed pleural metastasis,primary lesions,and distal extra-pleural metastasis. Results Of the 59 patients who were followed up for 5-18 months,23 had malignant hydrothorax,22 had benign hydrothorax,and 14 had hydrothorax with unknown etiology.The sensitivity and specificity of PET/CT were 73.9% and 81.8%,respectively,for pleural metastasis.PET/CT showed primary lesions and distal extra-pleural metastasis in 11 and 10 cases,respectively.PET/CT-guided biopsy was performed in 6 cases. Conclusion ¹⁸F-FDG PET/CT-guided biopsy has a promising prospect in differential diagnosis of hydrothorax.