Endoscopic ultrasound (EUS) has been established as an important modality for staging gastrointestinal malignancies. We have previously described the new technique of EUS-guided fine needle aspiration (FNA) in diagnosing submucosal and extraluminal tumors and lymph nodes. We now report the novel application of EUS and EUS-guided FNA in diagnosing malignant ascites and malignant pleural effusion in two patients with gastric carcinoma. In the first patient, CT did not detect any effusion or ascites. EUS-guided FNA diagnosed cytologically that both fluids were malignant, thereby precluding surgery. The second patient had a small pleural effusion on CT that was not detected on the chest x-ray. EUS-guided FNA made the cytological diagnosis of recurrent adenocarcinoma in the pleural fluid. These two cases demonstrate for the first time that EUS combined with EUS-guided FNA can be used for the diagnosis of malignant pleural or ascitic fluid in patients with gastrointestinal malignancies and might further extend the staging capabilities of EUS.