Pleural effusion is a common and important complication of malignancy which may at times be difficult to diagnose or treat. Its well recognized association with numerous diseases plus the limitations of our usual diagnostic tests may occasionally cause difficulty. In the oncology patient there are a number of common medical problems associated with the development of pleural effusion which frequently coexist with the malignancy. Pleural effusion may be a presenting or late sign of cancer, and when recurrent can be a vexing symptomatic problem. Fortunately, an increasing number of effective diagnostic and therapeutic modalities are available which, when judiciously applied, facilitate our approach.