Technetium-99m-methylene diphosphonate (MDP) uptake in a sympathetic effusion: an index of malignancy and a review of the literature. We report a patient with a sympathetic pleural effusion secondary to T-cell lymphoma that accumulated the bone imaging agent, Technetium-99m-methylene diphosphonate (99mTc-MDP). This case is significant in that malignant cells were not present on three cytologic examinations of the pleural fluid or multiple pleural biopsies. We also present a review of the published literature on pleural effusions that accumulate bone tracers. We conclude that pleural effusions that accumulate 99mTc-MDP should be considered malignant or secondary to a malignancy and further work-up is essential even if the cytologic exam of the pleural fluid is unremarkable. 