Originally, it was described as an uncommon finding in malignant pleural effusion and malignant ascites (2), occasionally seen prior to detection on chest radiograph. It was seen in patients with carcinoma of the breast, ovary, and lung most commonly. However, more recent studies have evaluated benign and malignant pleural effusions and their relation to 99m-Tc-MDP uptake. In a study by Kosuda et al (3), it was found that 61% of all patients with a pleural effusion demonstrated uptake of radiotracer (n=76), with no significant discrepancy between benign (57% of all benign) and malignant (62% of all malignant) effusions. They also concluded that uptake was directly proportional to the volume of the effusion, by a mechanism of passive diffusion. A further study describes a sensitivity of 40% and a specificity of 80% for malignancy, making bone scan suggestive of, but not diagnostic of, malignant pleural effusion (1). Sandler et al (1) suggests a mechanism of increased capillary permeability, likely secondary to invasion of the visceral pleura by tumor in the case of malignant effusion. The pleural fluid has not been found to contain free TCO4- or other metabolites.